Musnad Abu Ya'la al-Mawsili 129:121

Zuhayr narrated to us, Yazid bin Harun narrated to us, Sulayman al-Taymi informed us from al-Hasan, and Humayd informed us from Anas, both of whom said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Support your brother, whether he is an oppressor or oppressed." They said: "O Messenger of Allah, we support him when he is oppressed, but how do we support him when he is an oppressor?" He said: "You restrain him from oppression."