Sahih Muslim Book 32, Hadith Number 6254.
Chapter : Help your brother whether he is an oppressor or an oppressed one.
Jabir b. Abdullah reported that two young men, one from the Muhajirin (emigrants) and the other one from the Angr (helpers) fell into dispute and the Muhajir called his fellow Muhajirin, and the Ansari (the helper) called the Ansar (for help). In the meanwhile, Allah’s Messenger (may peace be upon him) came there and said: What is this, the proclamation of the days of jahiliya (ignorance)? They said: Allah’s Messenger, there is nothing serious. The two young men fell into dispute and the one struck at the back of the other. Thereupon he (the Holy Prophet) said: Well, a person should help his brother whether he is an oppressor or an oppressed. If he is the oppressor he should prevent him from doing it, for that is his help; and if he is the oppressed he should be helped (against oppression).