Know that these two matters are from the most vilest and despicable of things, yet the most widely spread amongst mankind, such that no one is free from them except for a few people. Thus, I begin with these two, due to the public's need for being warned of them.
As for Backbiting (gheebah), then it is when you mention something about a person (in his absence), that he hates (to have mentioned), whether it is about: His body, his religious characteristics, his worldly affairs, his self, his physical appearance, his character, his wealth, his child, his father, his wife, his servant, his slave, his turban, his attire, his manner of walking, his smile, his dissoluteness, his frowning, his cheerfulness or anything else related to the above. Likewise, it is the same whether you mention that about him with words, through writings, or whether you point or indicate him by gesturing with your eyes, hand or head.
As for the body, then it is when you say: "he is blind", "he limps", "he is bleary-eyed", "he is bald", "he is short", "he is tall", "he is black", "he is yellow". As for his religious qualities, then it is when you say: "he is a sinner", "he is a thief", "he is a betrayer", "he is an oppressor", "he takes the prayer lightly", "he is lenient with impurities", "he does not behave well towards his parents", "he does not pay the Zakaat duly", and "he does not avoid backbiting." As for the worldly matters, then it is when you say: "he has poor manners", "he's unmindful of people", "he does not think that anyone has a right over him", "he talks too much", "he eats and sleeps too much", "he sleeps at the wrong times", "he sits in places he does not belong in."
As for those matters connected with one’s parents, then it is when you say: "his father is a sinner", "an Indian", "a Nabatean", "a Negro", "a loafer", "a seedsman", "a cattle dealer", "a carpenter", "a blacksmith", "a weaver." As for his character, then it is when you say: "he has bad manners", "he is arrogant", "he is quarrelsome", "he is rash and hasty", "he is tyrannical", "he is feeble", "he has a weak heart", "he is irresponsible", "he is dismal", "he is dissolute", etc. As for the garment: "it has wide sleeves", "it has short hems", "what a filthy garment", and so on.
The remaining categories can be deduced based upon what we have mentioned, considering that the source rule behind it is: “Mentioning some aspect about him that he hates to have mentioned.” Imaam Abu Haamid Al-Ghazaalee quoted the consensus of the Muslim scholars with regard to backbiting being: “One's mentioning something about other people (in their absence), which they hate to have mentioned.” The authentic hadeeth clarifying this will be stated later.
As for Gossiping (nameemah), then it is when one conveys and carries the talk of people from one group of individuals to another with the intent of causing discord between the two of them.
These are the definitions of the two. As for their rulings, then they are forbidden according to the unanimous agreement of the Muslim scholars. The clear evidences in the Qur’aan, the Sunnah and the consensus of the ummah indicate its prohibition. Allaah says:
"And do not backbite one another." [Surah Al-Hujuraat: 12]
And He says:
"Woe to every slanderer and backbiter." [Surah Al-Humazah: 1]
And He says:
"A slanderer going about with gossip." [Surah Al-Qalam: 11]
Hudhayfah (رضي الله عنه) reported that the Prophet (صلى الله علیه وسلم) said:
"The one who spreads gossip (Namaam) will not enter Paradise." 
Ibn 'Abbaas (رضي الله عنه) reported that Allaah’s Messenger once passed by two graves and said:
"Verily, they (occupants of graves) are both being tormented and they are not being tormented for something major."
In the report from Al-Bukhaaree, it states:
"Rather, it is indeed something major. As for the first one, then he used to go around spreading gossip (nameemah) and as for the other, then he used not to protect himself from his own urine." 
The scholars say the meaning of "and they are not being tormented for something major" is "something major" according to their opinion or "something major" for them to have abandoned doing.
Abu Hurairah (رضي الله عنه) reported that Allaah’s Messenger (صلى الله علیه وسلم) once said (to his Companions):
"Do you know what backbiting is?" They said: "Allaah and His Messenger know best." He said: "Your mentioning something about your brother (in his absence) that he hates (to have mentioned)." It was said: "What if there exists in my brother, that which I say (of him)?" He (صلى الله علیه وسلم) said: "If there is found in him what you say, then you have backbitten him. And if there is not found in him what you say, then you have slandered him." 
Abu Bakrah (رضي الله عنه) reported that Allaah’s Messenger (صلى الله علیه وسلم) said in his sermon on the day of Sacrifice, during his farewell pilgrimage:
"Verily, your blood, your wealth and your honor are sacred for you, just as this day of yours is sacred, in this land of yours, in this month of yours. Have I not conveyed?" 
'Aa'ishah (رضي الله عنها) narrated:
"I said to the Prophet (صلى الله علیه وسلم): 'You are more than Safiyyah by such and such.' [Some of the narrators said it meant that she was short] So he (صلى الله علیه وسلم) said: 'You have stated a word, which if mixed with the water of the sea, it would surely stain it.'" 
At-Tirmidhee said this hadeeth was hasan saheeh. I say that the word for staining (mazaja) means: That it (her statement) would mix with the water with such a mixing that it would change its taste and smell due to the severity of its stench and vileness. This hadeeth is one of the greatest proofs in showing the forbiddance of backbiting, if not the greatest. And I do not know of any other hadeeth that reaches this level with regard to condemning backbiting.
“And he (the Prophet) does not speak from his own desire. Rather, it is only revelation revealed (to him).” [Surah An-Najm: 3-4]
We ask Allaah, the Most Generous, for His kindness and forgiveness from all detestable acts.
Anas (رضي الله عنه) reported that Allaah’s Messenger (صلى الله علیه وسلم) said:
“When I was ascended (to the heavens), I came across a people that had nails made of brass, with which they were scratching their faces and their chests. So I said: 'Who are these people, O Jibreel?' He said: 'They are the ones who eat the flesh of people  and dishonor them.'” 
It is also reported from Sa'eed Ibn Zayd (رضي الله عنه) that the Prophet (صلى الله علیه وسلم) said:
“Verily, one of the worst forms of Ribaa (interest) is being condescending with regard to a Muslim's honour, without due right.” 
Abu Hurairah (رضي الله عنه) reported that the Messenger of Allaah (صلى الله علیه وسلم) said:
“A Muslim is the brother of another Muslim. He does not betray him, nor does he lie to him, nor does he forsake him. All of the Muslim to (another) Muslim is sacred – his honor, his wealth and his blood. Taqwaa (fear and obedience to Allaah) is right here [pointing to his chest]. It is enough evil for a person to look down upon his fellow Muslim.” 
At-Tirmidhee said it was a hasan hadeeth. I say that there is no hadeeth greater in advantage and more numerous in points of benefit than this hadeeth. And with Allaah lies the success.
 Saheeh – Reported by Al-Bukhaaree (10/472 of al-Fath) and Muslim (105), and the wording is from him.
 Saheeh – Reported by Al-Bukhaaree (1/317 and 322, 3/222-223 and 242) and Muslim (292)
 Saheeh - Reported by Muslim (2589)
 Saheeh – Reported by Al-Bukhaaree (1/199 of al-Fath)
 Saheeh – Reported by Abu Dawood (4875), At-Tirmishee (2624 of at-Tuhfah) and Ahmad (6/189) from the path of Sufyaan Ibn ‘Alee Ibn Al-Aqmar. I say: Its chain of narration is authentic; its narrators are reliable.
 Translator’s Note: This is a reference to backbiting, since Allaah likens it to eating a human’s flesh in His saying:
“And do not backbit one another! Would any of you like to eat the flesh of his dead brother? You would surely hate it!” [Surah Al-Hujuraat: 12]
 Saheeh – Reported by Ahmad (3/224), Ibn Abee Ad-Duniyaa in as-Samat (165 and 572) from Abul-Mugheerah. It has also been reported by Abu Daawood (4878-4879). In summary, the hadeeth in mawsool form is authentic and Allaah knows best.
 Saheeh – Reported by Abu Daawood (4876), Ahmad (1/190) and Al-Haytham Ibn Kulaib in al-Musnad (2/30) from the path of ‘Abdullaah Ibn Abee Husayn from Nawfil Ibn Masaahiq. I say: This chain of narration is authentic – all of its narrators are reliable. The hadeeth has supporting evidences from other ahaadeeth reported by Al-Baraa Ibn ‘Aazib, ‘Abdullaah Ibn Mas’ood and ‘Abdullaah Ibn ‘Abbaas ((رضي الله عنه)m). See at-Targheeb (3/503-505).
 Saheeh – Reported by At-Tirmidhee (1992) from the path of Hishaam Ibn Sa’ad from Zayd Ibn Aslam from Abu Saalih. At-Tirmidhee declared it hasan, and it is as he said. It has another path of narration reported by Muslim (2564) and Ahmad (2/277, 311 and 360) from Abu Sa’eed. The author (رحمه الله) forgot to mention these paths of narration.