Musnad Ahmad 788:192

Yazid ibn Harun narrated to us, Sulayman informed us, from al-Hasan, from Abu Musa, from the Prophet, peace and blessings of Allah be upon him: "If two Muslims confront each other with their swords, and one of them kills his companion, then both of them are in the Fire." It was said: "O Messenger of Allah, this is the killer, but what is the case of the killed?" He said: "He was intending to kill his companion."