Musnad Ahmad 830:52

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Mansur, from Rib'i ibn Hirash, 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 upon the brink of Hell. If one of them kills his companion, they both enter it."