Sahih Muslim
The Book Pertaining to the Turmoil and Portents of the Last Hour (Kitab Al-Fitan wa Ashrat As-Saah)
Book Number: 041 / Hadith Number: 6901
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.