49:9
وَإِن طَآئِفَتَانِ مِنَ ٱلْمُؤْمِنِينَ ٱقْتَتَلُوا۟ فَأَصْلِحُوا۟ بَيْنَهُمَا ۖ فَإِنۢ بَغَتْ إِحْدَىٰهُمَا عَلَى ٱلْأُخْرَىٰ فَقَٰتِلُوا۟ ٱلَّتِى تَبْغِى حَتَّىٰ تَفِىٓءَ إِلَىٰٓ أَمْرِ ٱللَّهِ ۚ فَإِن فَآءَتْ فَأَصْلِحُوا۟ بَيْنَهُمَا بِٱلْعَدْلِ وَأَقْسِطُوٓا۟ ۖ إِنَّ ٱللَّهَ يُحِبُّ ٱلْمُقْسِطِينَ
And if two factions among the believers should fight, then make settlement between the two. But if one of them oppresses the other, then fight against the one that oppresses until it returns to the ordinance of Allah. And if it returns, then make settlement between them in justice and act justly. Indeed, Allah loves those who act justly.
Translation · Saheeh International
Tafsir
Ibn Kathīr (abridged)
Making Peace between Disputing Muslims
Allah commands making peace between Muslims that fight each other,
وَإِن طَآئِفَتَانِ مِنَ الْمُؤْمِنِينَ اقْتَتَلُواْ فَأَصْلِحُواْ بَيْنَهُمَا
(And if two parties among the believers fall to fighting, then make peace between them both.) Therefore, Allah calls both opposing groups among Muslims, believers, although they are fighting each other. Al-Bukhari and other scholars relied on this Hadith as evidence that committing a sin does not nullify faith, no matter how major the sin is. This creed contradicts the creed of the Khawarij sect and those who accepted their idea, such as the Mu`tazilah sect. Al-Bukhari narrated that Al-Hasan said that Abu Bakrah said that the Messenger of Allah ﷺ gave a speech on the Minbar while Al-Hasan bin `Ali was with him. He was repeatedly looking at Al-Hasan and then at the people; then said,
«إِنَّ ابْنِي هذَا سَيِّدٌ وَلَعَلَّ اللهَ تَعَالَى أَنْ يُصْلِحَ بِهِ بَيْنَ فِئَتَيْنِ عَظِيمَتَيْنِ مِنَ الْمُسْلِمِين»
(Verily, this son of mine is a Sayyid (chief or master), and may Allah make peace between two great groups of Muslims through him.) What the Prophet said, occurred. Al-Hasan brought peace between the people of Ash-Sham and `Iraq, after they fought tremendous wars and frightening battles. Allah's statement,