Knowledge of the Religion (Deen) of Islaam with the proofs. It is to submit to Allaah with tawheed, and to yield obediently to Him, and to free and disassociate oneself from shirk and its people. And its is of three levels: Islaam (submission and obedience to Allaah), eemaan (true faith comprising belief in the heart, speech of the tongue and action of the limbs), and ihsaan (perfection of worship). Each level has its pillars.
The First Level
The pillars of Islaam are five: The testification that none has the right to be worshipped except Allaah, and that Muhammad is the Messenger of Allaah; to establish the Prayer; to pay the zakaat; to fast Ramadaan; and to make hajj to the sacred House of Allaah.
So the proof for the testification (shahaadah) is the Saying of Allaah, the Most High,
"Allaah bears witness that none has the right to be worshpped but Him; and likewise the angels and the people of knowledge bear witness: He Who maintains justics, none has the right to be worshipped but Him, the All-Mighty, the All-Wise." [Soorah Aal-'Imraan (3):18]
Its meaning is that none has the right to be worshipped except Allaah: "laa ilaaha" (Nothing has the right to be worshipped besides Allaah), and "illallaah" (except Allaah) affirms worship for Allaah alone, and that there is to be no one given any share of His Dominion and Sovereignty.
The explanation which will make it clear is the Saying of Allaah, the Most High,
"And remember when Ibraaheem said to his father and his people: 'I am totally free from everything that you worship except for the one who created me. He will guide me upon the true Religion and the way of right-guidance.' And Allaah made this saying, that none has the right to be worshipped except Allaah, to persist amongst Ibraaheem's progeny, so that they might remember and return to obedience to their Lord, and to worshipping Him alone, and repent from their unbelief and their sins." [Soorah az-Zukhruf (43):26-28]
And His Saying:
"Say: O People of the Book, come to a word of justice between us, that we will single Allaah out with all worship and will not worship anything besides Him, and will disassociate ourselves from everything that is worshipped besides Him. Nor will we take one another as Lords besides Allaah by obeying one another in that which involves disobedience to Allaah. So if they turn away, then say: 'Bear witness that we are Muslims, submitting to Allaah and making our worship purely and sincerely for Him, and not worshipped anything else besides Him.'" [Soorah Aal-'Imraan (3):64]
The proof for the testification that Muhammad is the Messenger of Allaah, is the Saying of Allaah, the Most High,
"There has indeed come to you Allaah's Messenger, from amongst yourselves and known to you. It grieves him that you should suffer. He is eager and anxious for the guidance of those of you who are astray, and that they should repent and return to the truth, and he is full of compassion and mercy for the Believers." [Soorah at-Tawbah (9):128]
The meaning of the testification that Muhammad is the Messenger of Allaah is: to obey him in whatever he commands; to believe and testify to the truth of everything he informs of; to avoid whatever he forbade and prohibited; and that you worship Allaah only with that which he prescribed.
The evidence for the Prayer (as-salaat) and the zakaat, and the explanation of tawheed is the Saying of Allaah, the Most High,
"And they were not commanded except that they should worship Allaah alone, making their worship and obedience purely for Him, upon the true Religion and free from shirk; and that they should establish the prayer, and pay the zakaat - and that is the straight and true religion." [Soorah al-Bayyinah (98):5]
The evidence for Fasting (siyaam is the Saying of Allaah, the Most High,
"O you who believe Fasting is prescribed as an obligation for you as it was prescribed as an obligation to those who came before you, so that you may attain taqwaa (obedience to Allaah and avoidance of whatever He has forbidden)." [Soorah al-Baqarah (2):183[
The evidence for hajj is the Saying of Allaah, the Most High,
"And hajj to Allaah's sacred Hous is an obligation upon those who are able to perform it; and whoever refuses and rejects the obligation of hajj to Allaah's House, then Allaah has no need of him or of any of the creation." [Soorah Aal-'Imraan (3):97]
The Second Level
Eemaan - and it has seventy and odd branches, the highest of them is the saying that 'none has the right to be worshipped except Allaah' (laa ilaaha illallaah), the lowest of them is removal of that which is harmful from the path, and a sense of shame (al-hayaa) is a branch of eemaan. Its pillars are six: to truly believe in Allaah; His Angels; His Books; His Messengers; the Last Day; and that you truly believe in pre-decree (al-qadar) its good and evil. The proof for these six pillars is the Saying of Allaah, the Most High,
"It is not righteousness that you turn your faces to the east or the west, but rather righteousness is the righteousness of those who truly believe in Allaah, and the Last Day, and the Angels, and the Books, and the Prophets." [Soorah al-Baqarah (2):177]
The proof for pre-decree is the Saying of Allaah, the Most High,
"We have created all things in accordance with a pre-decreed measure." [Soorah al-Qamar (54):49]
The Third Level
Al-Ihsaan [lit. to do well or perfectly], it is a single pillar which is: that you worship Allaah as if you were seeing Him, and even though you do not see Him then He certainly sees you. The proof is the Saying of Allaah, the Most High,
"Allaah is with those who fear Him and keep away from what He has forbidden, and those who are people of ihsaan - those who do well in carrying out whatever He has obligated, taking care of His rights and being constant in obedience to Him, He aids, guides and assists them." [Soorah an-Nahl (16):128]
And His Saying,
"And place your reliance O Muhammad in the All-Mighty, the Bestower of Mercy. He Who sees you when you stand to pray, amd sees your movements alongs with those who follow you in the Prayer - in your standing, bowing, prostration and sitting. Your Lord is the One who hears whatever you recite and mention in your Prayer; and Who knows whatever you and those following you do in your Prayer - so recite the Qur'aan in it, and correctly perform it, since your Lord sees and hears you." [Soorah ash-Shooraa (26):217-220]
And His Saying,
"You are not involved in any matter; O Muhammad, nor do you recite the Book of Allaah, nor do you do any action - O people - whether it is good or evil, except that We are witnessing your deeds when you do them..." [ Soorah Yoonus (10):61]
The proof from the sunnah is the well-known hadeeth of Jibraa'eel, reported from 'Umar, radi-Allaahu 'anhu, that he said,
"Whilst we were sitting in the presence of Allaah's Messenger (sal-Allaahu 'alayhe wa sallam) one day a man came to us having very white clothes and very black hair. No trace of having travelled was to be seen upon him, nor did any of us know him. So he came and sat down with the Prophet (sal-Allaahu 'alayhe wa sallam) and put his knees against his knees, and placed his palms upon his thighs and said: "O Muhammad, inform me about Islaam." So Allaah's Messenger (sal-Allaahu 'alayhe wa sallam) said: "Islaam is that you testify that none has the right to worshipped except Allaah, and that Muhammad is the Messenger of Allaah; establish the Prayer; pay the zakaat; fast Ramadaan; and perform pilgrimage (hajj) to the House if you are able to do so." He said: "You have spoken the truth." So we were amazed at him asking him a question and then saying that he had spoken the truth. He said: "Then inform me about eemaan." He said: "It is that you truly believe in Allaah, His Angels, His Books, His Messengers, the Last Day; and that you truly believe in pre-decree - its good and its bad." He said: "You have spoken the truth." He said: "Then inform me about al-ihsaan." He said: "It is that you worship Allaah as if you were seeing Him, and though you do not see Him then He certainly sees you." He said: "Then inform me of the (Last) Hour." He said: "The one who is asked about it knows no better than the one who is asking." He said: "Then inform me about its signs." He said: "That the slave-girl will give birth to her mistress; and that you will see the barefooted, unclothed and destitute shepherds competing in the building of tall buildings." He said: so he left, and we remained for some time, then he (sal-Allaahu 'alayhe wa sallam) asked: "O 'Umar, do you know who the questioner was?" I replied: "Allaah and His Messenger know best." He said: "That was Jibraa'eel, he came to you to teach you your Religion (Deen)." 
 Related in Saheeh Muslim (1/1-3). For a proper explanation of this hadeeth, refer to Majmoo' al-Fataawaa war-Rasaa’il (3/143) of Shaykh Muhammad Ibn Saalih al-'Uthaymeen.