It is narrated from Hazrat Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: Help your brother, whether he is an oppressor or the oppressed. Someone asked, O Messenger of Allah (peace and blessings be upon him)! It is understandable to help the oppressed, but how should we help the oppressor? He replied: Stop him from committing oppression; that is his help.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 13079