Jabir bin Abdullah reported,

Two boys got into a fight, one from the Emigrants and one from the Helpers. The Emigrant, or the Emigrants, called out, "Help, Emigrants!" And the Helper called out, "Help, Helpers!"The Messenger of Allah ﷺ came out and said, "What is this, the call of the people of ignorance?"They said, "No, Messenger of Allah, it is only that two boys fought and one kicked the other from behind."He said, "There is no harm in that. Let a man support his brother, whether he is the wrongdoer or the wronged. If he is the wrongdoer, let him restrain him, for that is supporting him. And if he is wronged, let him aid him."

Muslim 6582

Sahih

حدثنا أحمد بن عبد الله بن يونس حدثنا زهير حدثنا أبو الزبير

عن جابر قال

اقتتل غلامان غلام من المهاجرين وغلام من الأنصار فنادى المهاجر أو المهاجرون يا للمهاجرين ونادى الأنصاري يا للأنصار فخرج رسول الله ﷺ فقال ما هذا دعوى أهل الجاهلية قالوا لا يا رسول الله إلا أن غلامين اقتتلا فكسع أحدهما الآخر قال فلا بأس ولينصر الرجل أخاه ظالما أو مظلوما إن كان ظالما فلينهه فإنه له نصر وإن كان مظلوما فلينصره

مسلم 6582صحيح
Contents
Reference
Commentary
Ayat
Collection
Sahih Muslim
Book
Righteousness, maintaining ties, and good manners
Chapter
Helping a brother whether he is an oppressor or oppressed
Numbering
Darussalam 6582
Muslim 6582