Musnad Abu Ya'la al-Mawsili 160:137

Amr ibn Muhammad al-Naqid narrated to us, Abd al-Aziz ibn Muhammad al-Darawardi narrated to us, he said: Al-Ala’ ibn Abd al-Rahman informed me, from his father, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) was asked about backbiting (ghibah). He said: "Your mentioning your brother with what he dislikes." He said: "O Messenger of Allah, even if what I say is in my brother?" He said: "If it is in him, then you have backbitten him, and if it is not in him what you say, then you have slandered him."