Musnad Ahmad 830:145

Hashim narrated to us, al-Mubarak narrated to us, from al-Hasan, from Abu Bakrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If two Muslims confront each other with their swords, and both of them intend to kill their companion, then one of them kills the other, both of them are in the Fire." It was said: "O Messenger of Allah, this is the killer, but what is the matter with the killed one?" He said: "Because he intended to kill his companion."