The Messenger of Allah ﷺ was asked: Messenger of Allah! What is back-biting? He replied: It is saying something about your brother which he would dislike. He was asked again: Tell me how the matter stands if what I say about my brother is true? He replied: If what you say of him is true, you have slandered him, and if what you say of him is not true, you have reviled him. (Using translation from Abū Dāʾūd 4874)   

عَنِ النَّبِيِّ ﷺ أَنَّهُ قِيلَ لَهُ مَا الْغِيبَةُ؟ قَالَ «ذِكْرُكَ أَخَاكَ بِمَا يَكْرَهُ» قِيلَ وَإِنْ كَانَ فِي أَخِي مَا أَقُولُ؟ قَالَ «فَإِنْ كَانَ فِيهِ فَقَدِ اغْتَبْتَهُ وَإِنْ لَمْ يَكُنْ فِيهِ فَقَدْ بَهَتَّهُ»