Sahih Muslim Book - 45, Hadith - 6593

حَدَّثَنَا يَحْيَى بْنُ أَيُّوبَ ، وَقُتَيْبَةُ ، وَابْنُ حُجْرٍ ، قَالُوا : حَدَّثَنَا إِسْمَاعِيلُ ، عَنِ الْعَلاَءِ ، عَنْ أَبِيهِ ، عَنْ أَبِي هُرَيْرَةَ ، أَنَّ رَسُولَ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ، قَالَ: أَتَدْرُونَ مَا الْغِيبَةُ ؟ قَالُوا: اللَّهُ وَرَسُولُهُ أَعْلَمُ ، قَالَ: ذِكْرُكَ أَخَاكَ بِمَا يَكْرَهُ قِيلَ أَفَرَأَيْتَ إِنْ كَانَ فِي أَخِي مَا أَقُولُ ؟ قَالَ: إِنْ كَانَ فِيهِ مَا تَقُولُ ، فَقَدِ اغْتَبْتَهُ ، وَإِنْ لَمْ يَكُنْ فِيهِ فَقَدْ بَهَتَّهُ.

It was narrated from Abu Hurairah that the Messenger of Allah (s.a.w) said: "Do you know what backbiting is?" They said: "Allah and His Messenger know best." He said: "When you say about your brother something that he dislikes." They said: "What if what I say about my brother is true?" He said: "If it is true then you have backbitten him, and if it is not true then you have slandered him."