Sayyiduna Jabir (may Allah be pleased with him) said: Two young men, one from the Muhajirun and one from the Ansar, fought with each other. The Muhajir called out to the Muhajirun, and the Ansari called out to the Ansar. The Prophet (peace and blessings be upon him) came out and said: "What is this? Is it the call of the Jahiliyyah?" They said: "No, O Messenger of Allah! It is only that two young men fought, and one of them struck the other on the backside." He (peace and blessings be upon him) said: "There is no harm. A man should help his brother, whether he is the wrongdoer or the wronged. If he is the wrongdoer, let him stop him from wrongdoing, for that is his help (or he said a word like it); and if he is wronged, let him help him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20467