Anas (May Allah bepleased with him) reported:
Messenger of Allah ﷺ said, "Help your brother, whether he is an oppressor or is oppressed". A man enquired: "O Messenger of Allah! I help him when he is oppressed, but how can I help him when he is an oppressor?" He ﷺ said, "You can keep him from committing oppression. That will be your help to him".

[Al-Bukhari and Muslim].

وعنه قال: قال رسول الله ﷺ: “انصر أخاك ظالمًا أو مظلومًا” فقال رجل: يا رسول الله أنصره إذا كان مظلومًا أرأيت إن كان ظالمًا كيف أنصره؟ قال: "تحجزه -أو تمنعه- من الظلم فإن ذلك نصره” ((رواه البخاري)).