Musnad Ahmad 830:99

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Qatadah, from al-Hasan, from Abu Bakrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If two Muslims confront each other with their swords, and one of them kills his companion, then both the killer and the killed are in the Fire." They said: "O Messenger of Allah, this is the killer, but what is the matter with the killed?" He said: "He was intending to kill his companion."