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 فَقَالَ الْمُهَاجِرِيُّ يَا لَلْمُهَاجِرِينَ وَقَالَ الْأَنْصَارِيُّ يَا لَلْأَنْصَارِ فَخَرَجَ رَسُولُ اللهِ ﷺ فَقَالَ أَدَعْوَى الْجَاهِلِيَّةِ؟ فَقَالُوا لَا وَاللهِ إِلَّا أَنَّ غُلَامَيْنِ كَسَعَ أَحَدُهُمَا الْآخَرَ فَقَالَ لَا بَأْسَ لِيَنْصُرِ الرَّجُلُ أَخَاهُ ظَالِمًا أَوْ مَظْلُومًا فَإِنْ كَانَ ظَالِمًا فَلْيَنْهَهُ فَإِنَّهُ لَهُ نُصْرَةٌ وَإِنْ كَانَ مَظْلُومًا فَلْيَنْصُرْهُ