All believers are but brethren. Hence, [whenever they are at odds,] make peace between your two brethren, and remain conscious of God, so that you might be graced with His mercy.
Al-Hujurat (The Dwellings) Verse 10
Allah's Apostle said, "Help your brother, whether he is an oppressor or he is an oppressed one. People asked, "O Allah's Apostle! It is all right to help him if he is oppressed, but how should we help him if he is an oppressor?" The Prophet said, "By preventing him from oppressing others."
Sahih Bukhari: Volume 3, Book 43, Number 624