Maliks Muwatta Book 56, Hadith Number 10.

Malik related to me from al-Walid ibn Abdullah ibn Sayyad that al-Muttalib ibn Abdullah ibn Hantab al-Makhzumi informed him that a man asked the Messenger of Allah, may Allah bless him and grant him peace, “What is backbiting?” The Messenger of Allah, may Allah bless him and grant him peace, said, “It is to mention about a man what he does not want to hear.” He said, “Messenger of Allah! Even if it is true?” The Messenger of Allah, may Allah bless him and grant him peace, said, “If you utter something false, then it is slander.”