Musnad Ahmad 32:9

Hushaym narrated to us, he said: Ubayd Allah ibn Abi Bakr informed us, from Anas, and Yunus, from al-Hasan, they both said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Help your brother, whether he is an oppressor or oppressed." It was said: O Messenger of Allah, I help him when he is oppressed, but how do I help him when he is an oppressor? He said: "You restrain him, you prevent him, for that is his help."