What is this, the proclamation of the days of jahiliya (ignorance)? They said: Messenger of Allah, there is nothing serious. The two young men fell into dispute and the one struck at the back of the other. Thereupon he (the Holy Prophet) said: Well, a person should help his brother whether he is an oppressor or an oppressed. If he is the oppressor he should prevent him from doing it, for that is his help; and if he is the oppressed he should be helped (against oppression). (Using translation from Muslim 2584a)   

اقْتَتَلَ غُلَامَانِ غُلَامٌ مِنَ الْمُهَاجِرِينَ وَغُلَامٌ مِنَ الْأَنْصَارِ فَنَادَى الْمُهَاجِرِيُّ يَا لِلْمُهَاجِرِينَ وَنَادَى الْأَنْصَارِيُّ يَا لِلْأَنْصَارِ فَخَرَجَ رَسُولُ اللهِ ﷺ قَالَ مَا هَذَا؟ أَدَعْوَى الْجَاهِلِيَّةِ ؟ قَالُوا لَا يَا رَسُولَ اللهِ إِلَّا أَنَّ غُلَامَيْنِ اقْتَتَلَا فَكَسَعَ وَاحِدٌ مِنْهُمَا الْآخَرَ قَالَ فَلَا بَأْسَ وَلْيَنْصُرِ الرَّجُلُ أَخَاهُ ظَالِمًا أَوْ مَظْلُومًا إِنْ كَانَ ظَالِمًا فَلْيَنْهَهُ فَإِنَّهُ لَهُ نَصْرٌ أَوْ كَلِمَةً نَحْوَهَا وَإِنْ كَانَ مَظْلُومًا فَلْيَنْصُرْهُ