The Holy Prophet [s] said: "Slander acts quicker against the faith of a Muslim believer than leprosy does against his body."
Usul-I-Kafi, vol. 2, p. 257
Imam Amir-ul-Mu'mineen Mi [a] said: "The listener to backbiting is the same as the backbiter."
Ghurar-ul-Hikam, p. 307
The Holy Prophet [s] said: "The abandoning of backbiting is more valuable to Allah, Almighty and Glorious, than the performance of ten thousand rak'at of recommended prayers."
Bihar-ul-Anwar, vol. 75, p. 261
'Abd-ul-Mu'min-il-Ansari said that once he himself arrived in the presence of Imam Abil-Hassan Musa ibn Ja'far [a] where Abdillah-il-Ja'fari had attended and Abdul-Mu'min smiled at him. Then he [a] inquired whether Abdul-Mu'min liked Abdillah Al-Ja'fari when he replied to the Imam [a] that he did not like him ('Abdillah) but on his account (the seventh Imam). Then Imam [a] said: "He is your brother, and a Muslim believer is the brother of another Muslim believer though their fathers are not the same. Hence, cursed is he who accuses his (Muslim) brother, cursed is he who deceives his (Muslim) brother, cursed is he who does not advise his (Muslim) brother, and cursed is he who slanders his (Muslim) brother."
Bihar-ul-Anwar, vol. 75, p. 262
Imam Amir-ul-Mu'mineen Ali [a] said: "The most vicious person is he who looks for other people's defects while he overlooks his own faults."
Tara'if-ul-Hikam, p. 176
Imam Kazim [a] said: "Cursed is the one who backbites against one's brother (fellow Muslim)."
Bihar-ul-Anwar, vol. 74, p. 232