Musnad Ahmad 788:125

'Affan narrated to us, Hammam narrated to us, from Qatadah, Al-Hasan narrated to us, that Abu Musa al-Ash'ari had a brother who was called Abu Ruhm, and he used to be hasty regarding the tribulation (fitnah), and Al-Ash'ari used to dislike the tribulation. He said to him: "Were it not for what has reached me, I would not have told you that I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'There are no two Muslims who meet with their swords, and one of them kills the other, except that they both enter the Fire.'"