Jami` at-Tirmidhi 1934
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."
حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ مُحَمَّدٍ، عَنِ الْعَلاَءِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قِيلَ يَا رَسُولَ اللَّهِ مَا الْغِيبَةُ قَالَ ‏"‏ ذِكْرُكَ أَخَاكَ بِمَا يَكْرَهُ ‏"‏ ‏.‏ قَالَ أَرَأَيْتَ إِنْ كَانَ فِيهِ مَا أَقُولُ قَالَ ‏"‏ إِنْ كَانَ فِيهِ مَا تَقُولُ فَقَدِ اغْتَبْتَهُ وَإِنْ لَمْ يَكُنْ فِيهِ مَا تَقُولُ فَقَدْ بَهَتَّهُ ‏"‏ ‏.‏ قَالَ وَفِي الْبَابِ عَنْ أَبِي بَرْزَةَ وَابْنِ عُمَرَ وَعَبْدِ اللَّهِ بْنِ عَمْرٍو ‏.‏ قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ ‏.‏