Sahih Bukhari Volume 9, Book 85, Hadith Number 84.

Narrated By Anas : Allah’s Apostle said, “Help your brother whether he is an oppressor or an oppressed,” A man said, “O Allah’s Apostle! I will help him if he is oppressed, but if he is an oppressor, how shall I help him?” The Prophet said, “By preventing him from oppressing (others), for that is how to help him.”