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,
فَإِن بَغَتْ إِحْدَاهُمَا عَلَى الأُخْرَى فَقَـتِلُواْ الَّتِى تَبْغِى حَتَّى تَفِىءَ إِلَى أَمْرِ اللَّهِ
(But if one of them outrages against the other, then fight you (all) against the one that which outrages till it complies with the command of Allah.) means, until the rebellious group refers to the commands of Allah and His Messenger for judgement and they listen to and obey the truth. There is a Hadith in the Sahih in which Anas states that the Messenger of Allah ﷺ said,
«انْصُرْ أَخَاكَ ظَالِمًا أَوْ مَظْلُومًا»
(Help your brother, whether he is an oppressor or he is oppressed.) "I asked, `O Allah's Messenger! It is right that I help him if he is oppressed, but how should I help him if he is an oppressor' He said,
«تَمْنَعُهُ مِنَ الظُّلْمِ فَذَاكَ نَصْرُكَ إِيَّاه»
(By preventing him from oppressing others; this is how you help him in this case.)" Sa`id bin Jubayr said that tribes of Aws and Khazraj once got in a scuffle using date tree branches and slippers. Allah revealed this honorable Ayah commanding them to make peace between them. As-Suddi said, "A man from Al-Ansar, whose name was `Imran, had a wife called Umm Zayd. She wanted to visit her family, but her husband prevented her from visiting them by locking her in an upper room. So, none of her family could visit or see her. She sent someone to her family. They came, took her down from the room and wanted to take her away. Her husband was absent at the time, so his family called on their people. Their cousins came to help prevent the wife from going with her family. A push and shove situation occurred that led to them fighting using slippers. This Ayah was then revealed in their case, and the Messenger of Allah ﷺ sent someone to bring peace between them, and they both agreed to resort to the decision of Allah the Exalted." The statement of Allah the Exalted, next,
فَإِن فَآءَتْ فَأَصْلِحُواْ بَيْنَهُمَا بِالْعَدْلِ وَأَقْسِطُواْ إِنَّ اللَّهَ يُحِبُّ الْمُقْسِطِينَ
(Then if it complies, then make reconciliation between them justly, and be equitable. Verily, Allah loves those who are the equitable.) means, be fair in your judgement regarding the dispute that occurred between them,
إِنَّ اللَّهَ يُحِبُّ الْمُقْسِطِينَ
(Verily, Allah loves those who are the equitable.) Ibn Abi Hatim recorded, that `Abdullah bin `Amr said that the Messenger of Allah ﷺ said,
«إِنَّ الْمُقْسِطِينَ فِي الدُّنْيَا عَلَى مَنَابِرَ مِنْ لُؤْلُؤٍ بَيْنَ يَدَيِ الرَّحْمنِ عَزَّ وَجَلَّ بِمَا أَقْسَطُوا فِي الدُّنْيَا»
(Verily, those who are equitable in this life, will be on podiums made of pearls before Ar-Rahman, the Exalted and Most Honored, on account of their fairness in this life.) An-Nasa'i collected this Hadith. Allah's statement,
إِنَّمَا الْمُؤْمِنُونَ إِخْوَةٌ
(The believers are but a brotherhood.) means, all of them are brothers in Islam. The Messenger of Allah ﷺ said,
«الْمُسْلِمُ أَخُو الْمُسْلِمِ لَا يَظْلِمُهُ وَلَا يُسْلِمُه»
(The Muslim is the brother of the Muslim, he is not unjust with him nor does he forsake him.) In the Sahih,
«وَاللهُ فِي عَوْنِ الْعَبْدِ مَا كَانَ الْعَبْدُ فِي عَوْنِ أَخِيه»
(Allah helps the servant as long as the servant helps his brother.) Also in the Sahih:
«إِذَا دَعَا الْمُسْلِمُ لِأَخِيهِ بِظَهْرِ الْغَيْبِ قَالَ الْمَلَكُ: آمِينَ وَلَكَ بِمِثْلِه»
(If the Muslim invokes Allah for the benefit of his absent brother, the angel says afterwards, "Amin, and for you the same.") There are many Hadiths with this meaning, such as, in the Sahih,
«مَثَلُ الْمُؤْمِنِينَ فِي تَوَادِّهِمْ وَتَرَاحُمِهِمْ وَتَوَاصُلِهِمْ كَمَثَلِ الْجَسَدِ الْوَاحِدِ، إِذَا اشْتَكَى مِنْهُ عُضْوٌ تَدَاعَى لَهُ سَائِرُ الْجَسَدِ بِالْحُمَّى وَالسَّهَر»
(The parable of the believers in relation to the kindness, mercy and compassion they have for each other, is that of the body: when an organ of it falls ill, the rest of the body responds with fever and sleeplessness.) And also in Sahih.
«الْمُؤْمِنُ لِلْمُؤْمِنِ كَالْبُنْيَانِ يَشُدُّ بَعْضُهُ بَعْضًا»
(A believer to another believer is like a building whose different parts enforce each other.) The Prophet then clasped his hands with the fingers interlaced. Allah's statement,
فَأَصْلِحُواْ بَيْنَ أَخَوَيْكُمْ
(So make reconciliation between your brothers, ) refers to the two groups that fight each other,
وَاتَّقُواْ اللَّهَ
(and have Taqwa of Allah) in all of your affairs,
لَعَلَّكُمْ تُرْحَمُونَ
(that you may receive mercy.) and this is a promise from Allah that He will grant mercy to those who fear and obey Him.