He taught them what they needed to know with regard to the social behaviour with the people, such as kindness to parents, keeping ties of kinship, good companionship with friends and neighbours and other than this. He taught them how to conduct themselves in matters of buying and selling, pawning, leaving deposits, leasing, renting, giving and receiving donations, and other such matter, to such an extent that Aboo Dharr (رضي الله عنه) - said:
“Indeed the Messenger of Allaah (صلى الله علیه وسلم) passed away and there is not a bird flapping its wings in the sky, except that he mentioned to us some knowledge about it.” 
Foundations of the Deen
Salmaan al-Farsee - (رضي الله عنه) - narrated that it was said to him: Your Prophet has taught you everything, even how to relieve yourselves? So he said to them:
“Yes indeed! And he has prohibited us from facing the Qiblah (direction of Prayer) whilst relieving ourselves ...” 
Along with this, he also taught them the foundation of all these acts of worship, manners and social interactions.
This foundation being what the worshippers hold as ’aqeedah (belief) concerning Allaah - the deity who is worshipped and who alone deserves to be worshipped - regarding His Dhaat (Essence), His Names, His Attributes and His Actions; and what follows from this with regard to His universal Laws and His prescribed Laws, which is based upon His ultimate Wisdom and utmost Mercy. The Companions learnt all this from the Prophet (صلى الله علیه وسلم), clearly and without any ambiguity, and all this was built upon the complete Tawheed of Allaah (to single out Allaah alone for worship), having as its foundations two great pillars: Affirmation and Negation.
As for Affirmation: It is affirming what is due to Allaah - the Most High - from the rights of His Lordship, worship and His Names, Attributes and Actions.
As for Negation: Then it is negating any partners to Allaah - the Most High - in all that is due to Him.
The People of Sunnah
Those who followed in goodness (i.e. in ’aqeedah and manhaj) from those who reached the period of the Companions, or those who came after them from the Imaams of guidance - who deserved the Pleasure of Allaah - remained upon this path, as Allaah - the Most High - said:
“The first to embrace Islaam from the Muhaajireen (those who migrated from Makkah to al-Madeenah) and the Ansaar (those who lived in al-Madeenah and aided them after migration) and those who follow them in goodness (correct ’aqeedah and manhaj). Allaah is well-pleased with them, as they are well-pleased with Him. He has prepared for them Gardens beneath which rivers flow, to dwell therein forever. This is the supreme achievement.” [Soorah at-Tawbah (9):100]
Then they were followed by those who came later, being blind to the truth, or pretending to be blind to the truth. So they went astray and led others astray; either due to inabilities or shortcomings, or due to enmity and oppression. They innovated in the Deen of Allaah - the Most High - that which did not belong to it, in matters of ’aqeedah (beliefs), ’ibaadah (acts of worship) and sulook (manners). They did this by either altering the texts of the Book and the Sunnah; and if it was possible, they rejected them entirely!
Brief History of the Innovated Sects
Shaykhul-Islaam Ibn Taymiyyah (d.728H) said:
“And know that most of the innovations connected to uloom (sciences) and acts of worship occurred in this Ummah at the end of the period of the Rightly-Guided Khaleefahs; just as the Prophet (صلى الله علیه وسلم) informed, when he said:“Those of you who live long after me will see a peat deal of differences. So hold fast to my Sunnah and the Sunnah of the Rightly-Guided Khaleefahs after me.”
... So when the period of the Rightly-Guided Khaleefahs had passed and the rule of kingship appeared, deficiencies appeared in the leaders, and therefore deficiency spread also to the people of knowledge and Deen. So during the end of the rule of ’Alee - (رضي الله عنه) - the innovation of the Khawaarij [href]3></A> and Raafidah  appeared; this being connected to the issue of leadership and khilaafah, and what was connected to this from actions and Sharee’ah rulings.
The kingship of Mu’aawiyah was a kingship of mercy, so when it passed, the rule of Yazeed came and fitnah (trials and discord) took place within it: the killing of al-Husain in ‘Iraaq, the fitnah of the people of Hurrah in al-Madeenah and the siege of Makkah when ’Abdullaah Ibn az-Zubayr made his stand. Then Yazeed passed away and the Ummah split-up. Ibn az-Zubayr in the Hijaaz, Banu Hakam in ash-Shaam (Syria, Jordan and Palestine) and the jump to power of Mukhtaar Ibn Abee ’Ubayd and others in ‘Iraaq. All of this took place at the end of the period of the Companions, when there only remained the likes of ’Abdullaah Ibn ’Abbaas, ’Abdullaah Ibn ’Umar, Jaabir Ibn ’Abdullaah, Aboo Sa’eed al-Khudree and others. The innovation of the Qadariyyah  and the Mur‘jiah  then occurred and it was refined by those Companions who remained, as they had, along with others, refined the innovations of the Khawaarij and the Raafidah.
Indeed, the status of the first three generations is by agreement of the majority - the best and just generations. That which the Qadariyyah mostly spoke about was related to the actions of the servants, as did the Murji‘ah. Their speech was concerned with issues of obedience and disobedience; the Believer and the sinful-one; and other similar related matters such as the names and judgements, and promises and threats. Until then, they had not spoken about their Lord, nor about His Attributes, except in the last period of the lesser Taabi’oon, this being the last period of the Umayyah rule, when the third generation - the Atbaa’ut-Taabi’oon - began, and by which time most of the Taabi’oon had already passed away.
So the majority of the Companions had passed away by the end of the rule of the four Rightly-Guided Khaleefahs - until none remained from the people of Badr, except a few. The majority of the Taabi’oon had passed away in the last period of the younger Companions, during the reign of Ibn az-Zubayr and ’Abdul-Malik. The majority of the third generation passed away in the last part of the Umayyah rule, at the beginning of the ’Abbaasee rule.
It then occurred that many of the offices of authority and power started to be occupied by the non-Arabs, whilst at the same time, many Arabs left the offices of power. Some of the non-Arab books were translated into Arabic: the books of the Persians, the Indians and the Romans. Thus, what the Prophet (صلى الله علیه وسلم)
said, appeared:“Then lies will become widespread, to such an extent that a person will testify having been asked to do so, and will take an oath without being asked to do so.”
Then three matters occurred: raa‘ee (opinion), kalaam (philosophical speech and theology) and tasawwuf (Soofism). The innovation of the Jahmiyyah  also occurred - which is negating and denying the Attributes of Allaah - as well as its opposite: which is tamtheel (resembling Allaah to His creation) ...
Indeed, having knowledge of the foundations of things and its fundamental principles, and having knowledge of the Deen and its fundamentals; and the principles which emanate from it, is the greatest and most beneficial knowledge. Since, if a person does not encompass the knowledge of the reality of things - of which he is in need - keeps thorns in his heart.” 
A Brief Summary
Ibn al-Qayyim (d.751H) - (رحمه الله) said:
“The innovation of the Qadariyyah started at the end of the period of the Companions. So those that remained at that time, such as ’Abdullaah Ibn ’Umar, Ibn ’Abbaas, and their like, opposed and refuted it. Then the innovation of the Murji‘ah occurred, after the period of the Companions, so those of the major Taabi’oon who faced it, refined it. Then came the innovation of the Jahmiyyah after the passing away of the Taabi’oon. Its evil reached alarming proportions and the matter had become dire during the age of the Imaams such as Ahmad (d.241H) and those with him. There appeared after this, the innovation of hulool (incarnation), this appeared, with, and in the time of al-Hajjaaj.
So whenever Shaytaan brought about an innovation from these innovations, or from other than them, Allaah established from His Party and Army, those who refuted it and warned the Muslims against it - all for the sake of sincerity and sincere advice for Allaah, His Book, His Messenger and the people of Islaam.” 
Compilation of the ’Aqeedah
Al-Haafidh Ibn Hajr (d.852H) - (رحمه الله) - said:
“From that which occurred was: compilation of hadeeth, then tafseer, then the compilation of issues of fiqh (jurisprudence) emanating from pure opinion, and then the compilation of matters related to actions of the heart.
As for the first matter: it was opposed by ’Umar, Aboo Moosaa and a few others whilst the majority allowed it. As for the second: It was opposed by a group from the Taabi’oon, such as Imaam ash-Sha’bee (d.104H). As for the third: It was opposed by Imaam Ahmad (d.241H) and a small group. Imaam Ahmad’s opposition of this intensified.
Also from that which occurred was the compilation of the sayings about Usoolud-Deen (fundamentals of the Deen). Some embarked to affirm it (i.e. the Attributes of Allaah), whilst others negated it. The former went to the extent of making tasbbeeh (resembling Allaah to His creation), whilst the latter went to the extent of making ta’teel (denying the Attributes of Allaah). The rejection of this from the Salaf - such as Aboo Haneefah (d.l50H), Aboo Yoosuf(d.182H) and ash-Shaafi’ee (d.204H), and their sayings with regards to censuring the people of Kalaam (philosophical speech and theology) is well known. The reason for such censure was that the people of Kalaam spoke about those matters which both the Prophet (صلى الله علیه وسلم)
and his Companions remained quiet about. It is established from Maalik (d.179H) that there did not exist at the time of the Prophet (صلى الله علیه وسلم)
, nor that of Aboo Bakr or ’Umar, anything from these desires - meaning: the innovation of the Khawaarij, the Raafidah and the Qadariyyah. Indeed, those who came after the first three excellent generations expanded upon matters which the Imaams of the Taabi’oon and those who followed them, rejected.
The people of Kalaam did not content themselves, until they filled the Deen with issues and the sayings of the philosophers. They made this philosophy the basis and the foundational principle to which everything was referred back to, and all that which opposed it from the narrations (of the Prophet (صلى الله علیه وسلم), his Companions and the Salaf who followed them) then ta‘weel (false interpretation) was made of them, even if they were averse to the result. Nor did they content themselves with just this. They claimed that what they had compiled was the noblest branch of knowledge and the most deserving to be acquired; and that those who did not use what they had laid down, then they were from the laymen and the ignorant ones. So delight is for the one who clings to what the Salaf were upon, and distances himself from the innovations that the Khalaf (the late-comers who opposed the ’aqeedah and manhaj of the Salaf) introduced. However, if one cannot keep away from it, then let him take only that which he needs and let the way of the Salaf be his intended goal.” 
Refuting the Falsehood
It is from the ultimate wisdom of Allaah to make an opposition to the truth, so that by it, the correctness of the truth over falsehood can be made clear. Indeed, the purity of gold does not become apparent except by exposing it to fire. Allaah - the Magnificent and Majestic through His complete power, vast mercy and over-whelming strength has sent those who will disprove the evidences of the ones who oppose the truth, and who will refute and make clear the false doubts.
Imaam Ahmad (d.241H) - (رحمه الله) said:
“All praise is for Allaah who, in every age and intervals between the Prophets, raises up a group from the People of Knowledge, who call the misguidance to guidance and patiently bearing ill-treatment and harm. With the Book of Allaah they give life to the dead, and by Allaah’s Light they give sight to the blind. How many a person killed by Iblees have they revived. How many people astray and wandering have they guided. How beautiful their effect has been upon the people, and how vile people have been towards them. They expel from the Book of Allaah the alterations of those going beyond bounds, the false claims of the liars and the false interpretations of the ignorant-ones - those who uphold the banner of innovation and let loose the trials and discords, who differ about the Book, oppose the Book, and agree to oppose the Book. Those who speak about Allaah and His Book without knowledge, argue about what is ambiguous in the Book, and deceive the ignorant with such ambiguities. We seek refuge in Allaah from the trials of the misguided-ones.” 
 Saheeh: Related by Ahmad (5/153), at-Tiyaalasee (no.479) and at-Tabaraanee in al-Kabeer (no.1647). Its isnaad is saheeh.
 Related by Muslim (l/152) and Aboo Daawood (no.8).
 The Khawaarij were the first sect in Islaam to split from the way of the Prophet (صلى الله علیه وسلم)
and his Companions. They arose in the khilaafah of ’Alee, making khurooj (rebellion) against him, before the arbitration between him and Mu’aawiyah radiallaahu ’anhumaa. From their false ’aqeedah (beliefs) is: allowing rebellion against the legitimate Muslim ruler whether pious or wicked, and declaring a Muslim to be a kaafir (disbeliever) due to commission of a major sin. They were described by the Prophet ’alayhis-salaam as the Dogs of Hell-Fire. Refer to: Maqaalaatul-Islaamiyyeen (l/168) of Abool-Hasan al-Ash’aree, al-Bidaayah (8/22-44) of Ibn Katheer and Fathul-Baaree (12/282-302) of Ibn Hajr.
 The Raafidah (the rejecters) are an extreme sect of the Shee’ah who rejected Zayd Ibn ’Alee Ibn al-Husayn due to his refusal to condemn Aboo Bakr and ’Umar radiallaahu ’anhumaa. They rapidly deteriorated in ’aqeedah, morals and Deen - until the present day where their beliefs are those represented by the Ithna Ashariyyah Shee’ah of Iraan. From their false beliefs are: declaring all but three or five of the Companions to be disbelievers, the belief that their imaams have knowledge of the Unseen past, present and future, considering the imaamah to be one of the main pillars of eemaan (faith) and the incompleteness of the Qur‘aan. Refer to: Maqalaatul-Islaamiyyeen (l/65), al-Farq Baynal-Firaaq (no.21) of ’Abdul-Qaadir al-Baghdaadee and Talbees Iblees (pp.94.100) of Ibnul-Jawzee.
 The Qadariyyah are the followers of Ma’bad Ibn al-Juhannee, and from their false beliefs are: that Allaah has no prior knowledge of anything until it come into existence; that it is people and not Allaah who are the creators of their own actions, denying ’aqeedah in the Punishment of the Grave, and denying that the authentic hadeeth is an evidence for the ’aqeedah, unless it is related in mutawaatir form. Refer to al-Milal wan-Nihal (l/72) of ash-Shahrastaanee and Sawnul-Mantaqwal-Kalaam (p.160) of as-Suyootee.
 The Murji‘ah are those who reject that actions are a part of eemaan (faith), and they say that eemaan is affirmation of the heart and statement of the tongue only. The extreme from amongst them limit eemaan to belief of the heart only. They also deny that eemaan increases and decreases. Refer to al-Maqaalaat (l/214) and al-Farq Baynal-Fir’aaq (p.202).
 The Jahmiyyah are the followers of Jahm Ibn Safwaan, who unleashed upon this Ummah the horrific innovation of ta’teel (denial of Allaah’s Attributes) - either directly, or by twisting the meanings; such as twisting the meaning of Hand of Allaah to mean: His Power and Generosity. They also deny that Allaah is above His creation, above His Throne, as well as holding the belief that Hell-Fire are not ever-lasting. Refer to ar-Radd ’alal-Jahmiyyah by Imaam Ahmad and also ad-Daarimee and al-Ibaanah (p.141) of Abool-Hasan al-Ash’aree.
 Majmoo’ul-Fataawaa (10/354-368) of Ibn Taymiyyah.
 Tabdbeeb Sunan Abee Daawood (7/61) of Ibn al-Qayyim.
 Fathul-Baaree (13/253) of Ibn Hajar al-’Asqalaanee.
 Ar-Radd ’alal-Jahmiyyah waz-Zanaadiqah (p.2) of Imaam Ahmad.