The beloved Prophet Muhammad (Sallallaahu `alayhi wa sallam) 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.”
Sahih Bukhari Hadith No. 84 Book 85, Volume 009 Narrated by Anas (Radiyallaahu `anhu).