Anas Radiyallahu Anhu narrates that Rasullullah Sallallahu Alaihi Wasallam said:

Help your Muslim brother whether he is an oppressor or is oppressed.

A man asked: O Rasullullah! (Sallallahu Alaihi Wasallam) I will help him when he is oppressed, but how can I help him when he is an oppressor?

He (Sallallahu Alaihi Wasallam) replied: You stop or prevent him from oppression for indeed that is your help to him.

(Bukhari)