Musnad Abu Ya'la al-Mawsili 160:133

And with his chain of narration: That the Messenger of Allah, may Allah bless him and grant him peace, was asked about backbiting (ghibah), so he said: "It is that you say about your brother what he dislikes." It was said: "And if what I say about my brother is in him?" He said: "If what you say is in him, then you have backbited him, and if it is not in him, then you have slandered him."