Know that it is upon the one who hears a Muslim being backbitten to oppose it and prevent the one doing the backbiting. If he is not able to prevent him with his speech, then he should stop him with his hand. If he is neither able to do so with his hand or with his tongue, then he should get up and leave that gathering. And if he hears his teacher (shaykh) being backbitten – or anyone else who has a right over him, or if the person (being backbitten) is from the people of righteousness and nobility, then his concern with what we have mentioned above should be greater.
Abu ad-Dardaa (رضي الله عنه) narrated the Prophet (صلى الله علیه وسلم) said:
"Whoever turns away from (harming) the honour of his brother, Allaah will turn his face away from the Hellfire on the Day of Judgement." 
And it is reported in Saheeh Al-Bukhaaree and Muslim from 'Itbaan – or as some say ‘Utbaan – in his long and famous hadeeth, in which he narrated:
"The Prophet (صلى الله علیه وسلم) stood up to pray, so the people said: 'Where is Maalik Ibn ad-Dukhshum?' A man said: 'That hypocrite! Allaah and His Messenger do not love him.' So the Prophet (صلى الله علیه وسلم) said: 'Do not say that! Do you not see that he says: Laa Ilaaha Illaa Allaah (There is no deity worthy of worship except Allaah), desiring by that, the face of Allaah?' " 
And it is reported in Saheeh Muslim on Al-Hasan Al-Basree that 'Aa'idh Ibn 'Amr (رضي الله عنه), one of the Companions of Allaah’s Messenger, entered in the presence of 'Ubaydullaah Ibn Ziyaad and said: “O my son, indeed I heard the Messenger of Allaah (صلى الله علیه وسلم) say:
‘Truly, the most evil of caretakers (leaders) are the oppressive rulers, so beware of being one of them.’”
So he said to him:
“Sit down, for you are only from the worthless scraps of the Companions of Muhammad!” So he responded: “Was there any worthless scraps among them? Rather the worthless scraps only appeared after them in others.” 
Ka’ab Ibn Maalik (رضي الله عنه) reported in his long hadeeth concerning his repentance that the Prophet (صلى الله علیه وسلم) said while sitting with some people in Tabook: “What did Ka’ab Ibn Maalik do?” A man from Banu Salima said:
“O Allaah’s Messenger! The (beauty) of his cloak and a self-appreciation of his finery have allured and detained him (i.e. from going to fight in the expedition)!”
Upon this, Mu’aadh Ibn Jabal (رضي الله عنه) said to him:
“What an evil thing you have said! I swear by Allaah, O Messenger of Allaah, we do not know anything about him except good.” So Allaah’s Messenger (صلى الله علیه وسلم) remained silent. 
Jaabir Ibn ‘Abdillaah and Abu Talha ((رضي الله عنهما)) said:
“Allaah’s Messenger (صلى الله علیه وسلم) said: ‘No (Muslim) person abandons another Muslim in a place where his sanctity will be violated and his honor will be lowered, except that Allaah will abandon him in a place that he would love to have His support. And no (Muslim) person aids another Muslim in a place where his sanctity will be violated and his honor will be lowered, except that Allaah will aid him in a place that he would love to have His support.’ ” 
Mu’aadh Ibn Anas (رضي الله عنه) reported that the Prophet (صلى الله علیه وسلم) said:
“Whoever protects a believer from a hypocrite, Allaah will send forth an angel that will protect his flesh on the Day of Judgement from (being burned by) the fire of Hell. And whoever accuses a Muslim of anything, desiring to disgrace him, Allaah will withhold him over the bridge of the Hellfire until he takes back what he said.” 
 Hasan or Saheeh – Reported by At-Tirmidhee (1996), Ahmad (6/450), Ad-Dawlaabee in al-Kunnaa (1/124) and Ibn Abee ad-Duniyaa in as-Samat (250) from the path of Abu Bakr an-Nahshalee from Marzooq Ibn Abee Bakr At-Taymee from Umm ad-Dardaa. At-Tirmidhee said it was a hasan hadeeth. I say: It is as he said, meaning that he intended by this that the hadeeth has a weak chain, but it is reported in other paths, which have no defects, as he explains in the last part of his Sunan. So refer to it because it is important. This is because all of the narrators in the hadeeth are reliable except Marzooq.
“No one reported from him except Abu Bakr An-Nahshalee.”
However Al-Haafidh (Ibn Hajr) said in at-Tah-theeb (10/87):
“I think he is the one who came after.”
Then he said:
“Distinguishing: Marzooq, Abu Bukayr At-Tameemee Al-Koofee. He reported from Sa’eed Ibn Jubair, ‘Ikrimah and Mujaahid. And Laith Ibn Abee Sulaim, Israa’eel, ‘Umar Ibn Muhammad Ibn Zayd Al-‘Umaree, Ath-Thawree and Ash-Shareek reported from him. Ibn Hibbaan mentioned him in his book ath-Thiqaat (The Reliable). His place of origin is Koofah, but he resided in Rayy.”
And he (Ibn Hajr) said in his biography for him that he was reliable. Some students of knowledge misunderstand this from Al-Haafidh but there is no reason for this confusion, because Al-Haafidh (Ibn Hajr) thought that the first (Abu Bakr) was the second (Abu Bukair). So if these two names refer to one and the same person, as thought by Al-Haafidh, and this is what is clear to me, then the hadeeth is authentic. And if they are two separate people, then the hadeeth is hasan because Marzooq is acceptable when reported from by way of Shahr Ibn Haushab.
 Saheeh – Reported by Al-Bukhaaree (1/518, 2/157 and 172 and 323, 3/60-61, 11/241, 12/303 of al-Fath) and Muslim (33)
 Saheeh – Reported by Muslim (1830).
 Saheeh – Reported by Al-Bukhaaree (8/113-116) and Muslim (2769)
 Da’eef – Reported by Abu Dawood (4884), Ahmad (4/30), Al-Bayhaqee (8/167-168), Abu Nu’aim in al-Hilyah (8/189) and Ibn Abee ad-Duniyaa in as-Samat (241) from the path of Al-Laith Ibn Sa’ad. I say: This chain of narration is weak because Yahyaa Ibn Saleem and his shaykh, Ismaa’eel Ibn Basheer are both unknown.
 Hasan – Reported by Abu Dawood (4883), Ahmad (3/441), Al-Baghawee in Sharh As-Sunnah (13/105) and Ibn Abee ad-Duniyaa in as-Samat (248) from the path of Ibn Al-Mubaarak. I say: Its chain of narration is weak because Isma’eel Ibn Yahyaa al-Mi’aafaree is in it and he is unknown. However this hadeeth has supporting evidences that raise it to the level of being hasan (acceptable). Refer to them in at-Targheeb (515-520).
Important Note: Our Shaykh (Al-Albaanee) mentioned this hadeeth in Da’eef al-Jaami’-us-Sagheer (5/193) as being weak, but then declared it hasan (later) in Saheeh Sunan Abee Daawood (4086). The last grading is what is correct so know this.