Sahih al-Bukhari 2444
Narrated Anas:
Allah's Messenger (ﷺ) said, "Help your brother, whether he is an oppressor or he is an oppressed one. People 
asked, "O Allah's Messenger (ﷺ)! It is all right to help him if he is oppressed, but how should we help him if 
he is an oppressor?" The Prophet (ﷺ) said, "By preventing him from oppressing others."
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا مُعْتَمِرٌ، عَنْ حُمَيْدٍ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ انْصُرْ أَخَاكَ ظَالِمًا أَوْ مَظْلُومًا ‏"‏‏.‏ قَالُوا يَا رَسُولَ اللَّهِ هَذَا نَنْصُرُهُ مَظْلُومًا، فَكَيْفَ نَنْصُرُهُ ظَالِمًا قَالَ ‏"‏ تَأْخُذُ فَوْقَ يَدَيْهِ ‏"‏‏.‏