Allah the Exalted said:
رياض الصالحينAbū Zakariyyā Muḥyī al-Dīn Yaḥyā ibn Sharaf al-Nawawī٢٦٦ - باب تحريم سب المسلم بغير حق
٢٦٦ - باب تحريم سب المسلم بغير حق
And those who harm believing men and believing women for [something] other than what they have earned [i.e., deserved] have certainly borne upon themselves a slander and manifest sin.
- ﵁ - - ﷺ:
It is related from Ibn Masʿūd (may Allah be pleased with him) who said: The Messenger of Allah ﷺ said: "Reviling a Muslim is wickedness (fusūq), and fighting him is disbelief (kufr)." Agreed upon.
وعن أَبي ذرٍ - ﵁: أنهُ سَمِعَ رسُولَ اللهِ - ﷺ - يقول: «لاَ يَرْمِي رَجُلٌ رَجُلًا بِالفِسْقِ أَوِ الكُفْرِ، إِلاَّ ارْتَدَّتْ عَلَيْهِ، إنْ لَمْ يَكُنْ صَاحِبُهُ كذَلِكَ». رواه البخاري.
It is related from Abū Dharr (may Allah be pleased with him) that he heard the Messenger of Allah ﷺ saying: "No man accuses another of wickedness (fisq) or disbelief (kufr) but that it rebounds upon him, if his companion is not as he said." Reported by al-Bukhārī.
وعن أَبي هريرة - ﵁: أنَّ رسولَ اللهِ - ﷺ - قَالَ: «المُتَسَابَّانِ مَا قَالاَ فَعَلَى البَادِي منهُمَا حَتَّى يَعْتَدِي المَظْلُومُ ». رواه مسلم.
It is related from Abū Hurayrah (may Allah be pleased with him) that the Messenger of Allah ﷺ said: "For whatever two who revile each other say, the blame is upon the one who began it, so long as the wronged one does not transgress." Reported by Muslim.
وعنه، قَالَ: أُتِيَ النَّبيُّ - ﷺ - بِرَجُلٍ قَدْ شرِبَ قَالَ: «اضربوهُ» قَالَ أَبُو هريرةَ: فَمِنَّا الضارِبُ بيَدِهِ، والضَّارِبُ بِنَعْلِهِ، والضَّارِبُ بِثَوْبِهِ. فَلَمَّا انْصَرَفَ، قَالَ بَعْضُ القَوْمِ: أخْزَاكَ اللهُ! قَالَ: «لا تَقُولُوا هَذَا، لا تُعِينُوا عَلَيْهِ الشَّيْطَان». رواه البخاري.
It is also related from him, who said: A man who had drunk wine was brought to the Prophet ﷺ. He said: "Strike him." Abū Hurayrah said: So among us was the one who struck with his hand, the one who struck with his sandal, and the one who struck with his garment. When the man departed, some of the people said: May Allah disgrace you! He said: "Do not say this; do not help Satan against him." Reported by al-Bukhārī.
وعنه، قَالَ: سَمِعْتُ رسولَ اللهِ - ﷺ - يقول: «مَنْ قَذَفَ مَمْلُوكَهُ بِالزِّنَى يُقَامُ عَلَيْهِ الحَدُّ يَومَ القِيَامَةِ، إِلاَّ أَنْ يَكُونَ كَمَا قَالَ». متفق عَلَيْهِ.
It is also related from him, who said: I heard the Messenger of Allah ﷺ saying: "Whoever falsely accuses his slave of fornication will have the prescribed punishment carried out against him on the Day of Resurrection, unless it be as he said." Agreed upon.