جامع الترمذي

Abu Hurairah narrated that it was said: " O Messenger of Allah! What is backbiting?" He said : " Mentioning your brother with that which he does not like." He said " What if what I said about him is so?" He said: "If what you said about him is so, then you have backbitten him, and if it is not as you said, then you have slandered him."