Hadith

Anas (may Allah be pleased with him) reported that the Messenger of Allah (may the peace and blessings of Allah be upon him) said:

Help your brother whether he is an oppressor or is oppressed.

A man inquired:

O' Messenger of Allah! I may help him when he is oppressed, but how can I help him when he commits oppression?

He said:

You prevent him from committing oppression. That will be your help to him.

(Reported by Al-Bukhari)