Musannaf Ibn Abi Shayba 37:112

Yazid ibn Harun narrated to us, he said: Sulayman al-Taymi informed us, from Qatadah, from al-Hasan, from Abu Musa, who said: The Messenger of Allah (may Allah bless him and grant him peace) 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." They said: "O Messenger of Allah, this is the killer, but what is the case of the killed?" He said: "He intended to kill his companion."