Jabir ibn Abdullah reported: Two young men were fighting, one from the Muhajireen and one from the Ansar. The Muhajir called out, “O Muhajireen!” and the Ansari called out, “O Ansar!” The Messenger of Allah, peace and blessings be upon him, came out and he said, “What is this call of the people of ignorance?” They said, “O Messenger of Allah, it is nothing but two young men fighting and one hit the other from behind.” The Prophet said, “It does not matter. Let a man help his brother whether he is wrong or being wronged. If he is oppressing others, then stop him for that is helping him. If he is being oppressed, then help him.”

Source: Sahih Muslim 2584, Sahih Bukhari 2312

Grade: Muttafaqun Alayhi (authenticity agreed upon) according to Al-Bukhari and Muslim

عَنْ جَابِرٍ قَالَ اقْتَتَلَ غُلاَمَانِ غُلاَمٌ مِنَ الْمُهَاجِرِينَ وَغُلاَمٌ مِنَ الأَنْصَارِ فَنَادَى الْمُهَاجِرُ أَوِ الْمُهَاجِرُونَ يَا لَلْمُهَاجِرِينَ وَنَادَى الأَنْصَارِيُّ يَا لَلأَنْصَارِ فَخَرَجَ رَسُولُ اللَّهِ صلى الله عليه وسلم فَقَالَ مَا هَذَا دَعْوَى أَهْلِ الْجَاهِلِيَّةِ قَالُوا لاَ يَا رَسُولَ اللَّهِ إِلاَّ أَنَّ غُلاَمَيْنِ اقْتَتَلاَ فَكَسَعَ أَحَدُهُمَا الآخَرَ قَالَ فَلاَ بَأْسَ وَلْيَنْصُرِ الرَّجُلُ أَخَاهُ ظَالِمًا أَوْ مَظْلُومًا إِنْ كَانَ ظَالِمًا فَلْيَنْهَهُ فَإِنَّهُ لَهُ نَصْرٌ وَإِنْ كَانَ مَظْلُومًا فَلْيَنْصُرْهُ 

2584 صحيح مسلم كِتَاب الْبِرِّ وَالصِّلَةِ وَالْآدَابِ بَاب نَصْرِ الْأَخِ ظَالِمًا أَوْ مَظْلُومًا

2312 صحيح البخاري كِتَاب الْمَظَالِمِ انصر أخاك ظالما أو مظلوما