Surah Al-Hujuraat (49) — Ayah 9

9. And if two groups of the believers fight each other [10], then make peace [11] between them. But if one of them transgresses against the other, then fight against the one that transgresses [12] until it returns to the command of Allah. Then if it returns, make peace between them with justice and act justly. Surely, Allah loves those who act justly.

[10]
Fighting Amongst Yourselves is Not the Work of Believers:

From this sentence, it is understood that fighting is not the work of believers. This is also evident from the following hadiths:
(1) Sayyiduna Abu Bakr Siddiq ؓ narrates that he heard the Messenger of Allah ﷺ say: "When two Muslims confront each other with their swords, both the killer and the killed are in Hell." I asked: "This is clear for the killer, but what was the fault of the one who was killed?" He ﷺ replied: "He too was eager to kill his companion." [بخاری۔ کتاب الایمان۔ باب وان طائفتان۔۔ مسلم۔ کتاب الفتن۔ باب اذا توجہ المسلمان بسیفہما]
(2) Ahnaf bin Qais says that he went to help a certain person. (On the way) he met Abu Bakrah ؓ who asked: "Where are you going?" I said: "To help that person." Abu Bakrah ؓ said: "Go back, because I heard the Messenger of Allah ﷺ say: When two Muslims confront each other with their swords, both the killer and the killed are in Hell." I asked: "O Messenger of Allah, this is clear for the killer, but what was the fault of the one who was killed?" He ﷺ replied: "He too was eager to kill his companion." [بخاري۔ كتاب الايمان۔ باب المعاصي من امرالجاهلية مسلم۔ كتاب الفتن۔ باب اذا توجه المسلمان بسيفهما]
[11]
The Literal Meaning of "Taifah":

That is, despite these precautions, if two groups of Muslims fight, then make every effort to resolve their differences and bring about reconciliation between them. It should be noted that the word "taifah" is used here, and "taifah" generally refers to a small group. In fact, according to some commentators, this word can even apply to a single individual. That is, if there is a fight between two Muslims, the same ruling applies. And it is clear that the one who reconciles must be neutral, whether it is a single person or a group. Furthermore, it is necessary for the one reconciling to be able to influence both parties.
[12]
It is Necessary to Make Peace Among Muslims and Keep Justice in View:

That is, if a person alone does not have the ability to influence both parties, then you should do this work together; you should not remain sitting idle. First, review the situation and the disagreement, and see which party is on the truth and which is committing excess. Now, it is clear that a clap is never made with one hand; the party committing excess also has some right, and the one on the truth has also made some mistake. Therefore, first it must be determined where the greater right lies and who is the one committing excess. Then, the group reconciling should support the one with more right and thus put pressure on both parties for reconciliation. And so much pressure should be put on the one committing excess that he admits his excess, repents from it, and accepts the truth. When this stage is reached, then it should not be that a one-sided decision is given in favor of the party on the truth and an attempt is made to completely suppress the party committing excess. Rather, the decision should be made with justice and fairness, and if the party committing excess had some right, then those matters should also be considered at the time of decision. Because this is closer to the truth and is what Allah Almighty likes.

Shamas bin Qais the Jew Inciting Aws and Khazraj to Fight:

It should be noted that sometimes both parties are on the truth, and the cause of their fighting is some external enemy or outside interference. An example of this in the Prophetic era is that when the Muslims achieved a glorious victory in the Battle of Badr, the Jews of Madinah were greatly distressed by this. An old Jew, Shamas bin Qais, thought of solving this problem by rekindling war between Aws and Khazraj. For this purpose, he prepared a young Jew to go into the gatherings of the Ansar and bring up the topic of the Battle of Bu'ath, and to recite the poems that were said by Aws and Khazraj in that context, thus inflaming the spirit of pre-Islamic tribalism. When this Jew recited those poems, immediately the hostile emotions of the pre-Islamic era were stirred up between Aws and Khazraj, arguments began, and it reached the point where one party said to the other: If you wish, we can revive that war and settle it. Then the sounds of weapons began to be heard, and it was close to becoming a terrible war.

The Prophet's ﷺ Reconciliation Among Them:

The Messenger of Allah ﷺ was informed of this incident, so he immediately went to the scene with a few Muhajireen and said: "O Muslims! The call of ignorance in my presence! Allah Almighty has guided you to Islam and united your hearts. Then what is this matter now?" At that moment, they realized how they had fallen into the devil's trap. Their eyes were opened, and then the people of Aws and Khazraj began to embrace each other and weep.

Abdullah bin Ubay Inciting Ansar and Muhajireen to Fight in the Battle of Banu Mustaliq:

The second example is the discord that the hypocrite Abdullah bin Ubay stirred up between the Muhajireen and Ansar during the return journey from the Battle of Banu Mustaliq, inciting them to fight and kill each other. The details of this incident will come in Surah Munafiqun. On this occasion as well, the Messenger of Allah ﷺ immediately arrived at the scene and resolved the matter, bringing about reconciliation.

If Reconciliation is Not Possible, Then Remain Neutral:

And the greatest example of this verse is the wars between Sayyiduna Ali ؓ and Sayyiduna Amir Muawiyah. And the cause of these wars was a rebellious group. Since both these parties were politically very powerful, no such group could arise that could reconcile between them. Therefore, many of the noble Companions ؓ considered it best to remain neutral in these wars. As indicated in the second hadith above, Sayyiduna Abu Bakr Siddiq ؓ seems to be referring to these very wars. And in these wars, many Companions remained neutral and did not take part in the fighting.