Jami` at-Tirmidhi, Volume 4, Book 31, Hadith 2255 (DarusSalam) ⓘ

It was narrated from Anas bin Malik, that the Prophet(s.a.w) said: "Help your brother whether he is an oppressor or oppressed." It was said: "O Messenger of Allah! I help him when he is oppressed. But how can I help him when he oppresses?" He said: "Prevent him from oppression, that is your help for him."

حَدَّثَنَا مُحَمَّدُ بْنُ حَاتِمٍ الْمُكْتِبُ، حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ الأَنْصَارِيُّ، حَدَّثَنَا حُمَيْدٌ الطَّوِيلُ، عَنْ أَنَسٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏"‏ انْصُرْ أَخَاكَ ظَالِمًا أَوْ مَظْلُومًا ‏"‏ ‏.‏ قُلْنَا يَا رَسُولَ اللَّهِ نَصَرْتُهُ مَظْلُومًا فَكَيْفَ أَنْصُرُهُ ظَالِمًا قَالَ ‏"‏ تَكُفُّهُ عَنِ الظُّلْمِ فَذَاكَ نَصْرُكَ إِيَّاهُ ‏"‏ ‏.‏ قَالَ وَفِي الْبَابِ عَنْ عَائِشَةَ ‏.‏ قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ ‏.‏