كتاب البر والصلة والآداب
45  
The Book of Virtue, Enjoining Good Manners, and Joining of the Ties of Kinship
(16)
Chapter: Supporting One's Brother Whether He Is Doing Wrong Or Being Wronged
(16)
باب نَصْرِ الأَخِ ظَالِمًا أَوْ مَظْلُومًا ‏‏
Sahih Muslim 2584 a

Jabir b. Abdullah reported that two young men, one from the Muhajirin (emigrants) and the other one from the Angr (helpers) fell into dispute and the Muhajir called his fellow Muhajirin, and the Ansari (the helper) called the Ansar (for help). In the meanwhile, Allah's Messenger (ﷺ) came there and said:

What is this, the proclamation of the days of jahiliya (ignorance)? They said: Allah's Messenger, 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).
حَدَّثَنَا أَحْمَدُ بْنُ عَبْدِ اللَّهِ بْنِ يُونُسَ، حَدَّثَنَا زُهَيْرٌ، حَدَّثَنَا أَبُو الزُّبَيْرِ، 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 ‏"‏ ‏.‏
Reference : Sahih Muslim 2584a
In-book reference : Book 45, Hadith 80
USC-MSA web (English) reference : Book 32, Hadith 6254
  (deprecated numbering scheme)