Musnad Abu Ya'la al-Mawsili 160:98

Yahya ibn Ayyub narrated to us, Ismail narrated to us, he said: Al-Ala’ informed me, from his father, from Abu Hurairah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Do you know what backbiting (ghibah) is?" They said: "Allah and His Messenger know best." He said: "Your mentioning your brother with what he dislikes." It was said: "What if what I say is in my brother?" He said: "If what you say is in him, then you have backbitten him, and if it is not in him, then you have slandered him."