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

English translation · Jami' at-Tirmidhi