Musannaf Ibn Abi Shayba 37:277

Ghundar narrated to us, from Shu'bah, from Mansur, from Rib'i, from Abu Bakrah, from the Prophet (may Allah bless him and grant him peace), that he said: "When two Muslims bear arms against one another, they are on the brink of Hell. If one of them kills his companion, both of them shall enter it."