Someone asked, "Messenger of Allah, what is backbiting?"He replied, "It is mentioning your brother in a way he would dislike."The person asked, "What if what I say about my brother is actually true?"He answered, "If what you say about him is true, then you have backbitten him, and if it is not true, then you have slandered him."
حدثنا عبد الله بن مسلمة القعنبي حدثنا عبد العزيز يعني ابن محمد عن العلاء عن أبيه
أنه قيل يا رسول الله ما الغيبة قال ذكرك أخاك بما يكره قيل أفرأيت إن كان في أخي ما أقول قال إن كان فيه ما تقول فقد اغتبته وإن لم يكن فيه ما تقول فقد بهته
Muslim reported it in the Book of Righteousness and Maintaining Ties, Chapter on the Prohibition of Backbiting, hadith 2589, from the hadith of Ala bin Abdul-Rahman bin Yaqub through this route. Tirmidhi also reported it, hadith 1934, from the hadith of Abdul-Aziz Darawardi through this route.