Hadith on Backbiting: Speaking badly in their absence is backbiting

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 do you think if what I said about him is true?” 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