Sunan at-Tirmithiy كتاب البر والصلة باب ما جاء فى الغيبة
It was asked, "Messenger of God, what is backbiting?" He replied,"Mentioning your brother in a way he dislikes." The questioner then asked, "What if he is as I say?" He replied, "If he is as you say, then you have backbitten him; if he is not, then you have slandered him." Click on ḥadīth to read more