Mukhtasar Muslim (Abridged) 2005 — When two Muslims face each other

Ahnaf bin Qais says: I set out with the intention to join that person (i.e., to join Ali, may Allah be pleased with him, against Muawiyah, may Allah be pleased with him). On the way, Abu Bakr, may Allah be pleased with him, met me and said, "O Ahnaf, where are you going?" I said, "I want to help the cousin of the Messenger of Allah, peace and blessings be upon him." Abu Bakr, may Allah be pleased with him, said, "O Ahnaf! Return, for I heard the Messenger of Allah, peace and blessings be upon him, say: 'When two Muslims confront each other with their swords, both the killer and the one killed are in Hellfire.'" I said, or someone else said, "O Messenger of Allah! The killer will go to Hell, but why the one who is killed?" He, peace and blessings be upon him, said, "He also intended to kill his companion."