Sahih Bukhari Volume 009, Book 085, Hadith Number 084.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 85. Saying Something under Compulsion (Ikraah)
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."