Musnad Ahmad 830:146

Mu'ammal narrated to us, Hammad bin Zayd narrated to us, Ayyub, Yunus, Hisham, and Al-Mu'alla bin Ziyad informed us, from Al-Hasan, from Al-Ahnaf, from Abu Bakrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When two Muslims confront each other with their swords, and one of them kills his companion, then both of them are in the Fire."