Musnad Ahmad 788:106

Isma'il narrated to us, from Yunus, from al-Hasan, that a brother of Abu Musa used to hasten into tribulation (fitna). He began to forbid him, but he would not desist. He said: "I used to think that a little from me would suffice you," or he said, "from the admonition, less than what I see. And indeed, the Messenger of Allah (ﷺ) said: 'When two Muslims face each other with their swords, and one of them kills the other, then the killer and the killed are in the Fire.'" It was said: "This is the killer, but what is the matter with the killed?" He said: "He intended to kill his companion."