In the name of Allah, Most Gracious, Most Merciful.
Anas (May Allah be pleased with him) reported: The Messenger of Allah (SAWS) said, “Help your brother, whether his is an oppressor or is oppressed”. A man enquired: “O Messenger of Allah! I help him when he is oppressed, but how can I help him when he is an oppressor?” The Messenger of Allah (SAWS) said, “You can keep him from committing oppression. That will be your help to him”. [Al-Bukhari and Muslim].
SAWS: May Allah exalt his mention and protect him from imperfection
Riyaadiss-Saliheen (The Meadows of the Righteous)
By Imam An Nawawi
Chapiter 27, Page 81, No 237