The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wa'l-Adab)
Muslim :: Book 32 : Hadith 6306
'Abdullah b. Mas'ud reported that Muhammad (may peace be upon him) said: Should I inform you that slandering, that is in fact a tale-carrying which creates dissension amongst people, (and) he (further) said: The person tells the truth until he is recorded as truthful, and lie tells a lie until lie is recorded as a liar.