Abu Bakra reported the Prophet (ﷺ) as saying, “When two Muslims meet and one of them bears arms against his brother, they are both on the brink of hell, and if one of them kills the other they will both enter it.” In a version on Abu Bakra's authority he said, “When two Muslims meet with their swords, the one who kills and the one who is killed will go to hell." When he (i.e. Abu Bakr) remarked that one was the killer and asked what was wrong with the one who was killed, he replied, “He was eager to kill his companion." (Bukhari and Muslim.)