Musnad Ahmad 30:2773

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, he said: I heard al-'Ala' narrating from his father, from Abu Hurayrah, that he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Do you know what backbiting (al-ghibah) is?" He said: They said: Allah and His Messenger know best. He said: "Your mentioning your brother with that which is not in him." He said: What do you see if what I say about my brother is in him? He said: "If what you say is in him, then you have backbitten him, and if what you say is not in him, then you have slandered him."