Musnad Ahmad 788:265

Yazid narrated to us, he said: Sa'id informed us, from Qatadah, from al-Hasan, from Abu Musa, from the Prophet, peace and blessings of Allah be upon him, who said: "When two Muslims face 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 about the killed?" He said: "He intended to kill his companion."