The Prophet (may Allah's peace and blessings be upon him) said that one should support his brother and never disappoint him, whether he was an oppressor or oppressed. A man, then, said that helping one’s oppressed brother is by defending him if oppressed, but in case he is the oppressor, how can I help him? The Prophet (may Allah's peace and blessings be upon him) replied that he should help him by preventing him from oppressing others, which in reality is a help to him.