Sahih Muslim Book 41, Hadith Number 6901.

Chapter : When two Muslims confront each other with swords.

Abu Bakra reported Allah’s Messenger (may peace be upon him) as saying: When two Muslims (confront each other) and the one amongst them attacks his brother with a weapon, both of them are at the brink of Hell-Fire. And when one of them kills his companion, both of them get into Hell-Fire.