Musnad Ahmad 32:1136

Yazid narrated to us, Humaid informed us, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Support your brother, whether he is an oppressor or oppressed." It was said: "O Messenger of Allah, we support him if he is oppressed, but how do we support him if he is an oppressor?" He said: "You restrain him from oppression."