Riyad as-Salihin (The Meadows of the Righteous)
by Imam Nawawi
257. Chapter: On the prohibition against slander, which is carrying tales between people in order to create trouble
Allah says, "A backbiter, slandermonger," (68:11) and He says, "He does not utter a single word without a watcher by him, pen in hand." (50:18)
1536. Hudhayfa reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "A slanderer will not enter the Garden." [Agreed upon]
1537. Ibn 'Abbas reported that the Prophet, may Allah bless him and grant him peace, passed by two graves and said, "They are being punished and not for anything very great. One of them did not guard himself from urine and the other was involved in back-biting." [Agreed upon]
The scholars say the meaning of "not for anything very great" is as they claim.
1538. Ibn Mas'ud reported that the Prophet, may Allah bless him and grant him peace, said, "Shall I tell you what slander is? It is backbiting bandied about among people." [Muslim]