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كتاب المظالم
46
Oppressions
(4)
Chapter: Help your brother
(4)
باب أَعِنْ أَخَاكَ ظَالِمًا أَوْ مَظْلُومًا

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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 2444
In-book reference : Book 46, Hadith 5
USC-MSA web (English) reference : Vol. 3, Book 43, Hadith 624
  (deprecated numbering scheme)