Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said, “Do you know what backbiting is?” They said, “Allah and His Messenger know best.” The Prophet said, “To mention your brother in a way he dislikes.” It was said, “What if it is true about him?” The Prophet said, “If what you say about him is true, it is backbiting. If it is not true, it is slander.”
Source: Ṣaḥīḥ Muslim 2589
Grade: Sahih (authentic) according to Muslim
عَنْ أَبِي هُرَيْرَةَ أَنَّ رَسُولَ اللَّهِ 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 فَقَدْ اغْتَبْتَهُ وَإِنْ لَمْ يَكُنْ فِيهِ فَقَدْ بَهَتَّهُ
2589 صحيح مسلم كتاب البر والصلة والآداب باب تحريم الغيبة