Sahih al-Bukhari Book 87, Hadith 13
The Prophet (ﷺ) said, "Whoever carries arms against us, is not from us."
I was among those Naqibs (selected leaders) who gave the Pledge of allegiance to Allah's Messenger (ﷺ). We gave the oath of allegiance, that we would not join partners in worship besides Allah, would not steal, would not commit illegal sexual intercourse, would not kill a life which Allah has forbidden, would not commit robbery, would not disobey (Allah and His Apostle), and if we fulfilled this pledge we would have Paradise, but if we committed any one of these (sins), then our case will be decided by Allah.
I went to help that man (i.e., `Ali), and on the way I met Abu Bakra who asked me, "Where are you going?" I replied, "I am going to help that man." He said, "Go back, for I heard Allah's Messenger (ﷺ) saying, 'If two Muslims meet each other with their swords then (both) the killer and the killed one are in the (Hell) Fire.' I said, 'O Allah's Messenger (ﷺ)! It is alright for the killer, but what about the killed one?' He said, 'The killed one was eager to kill his opponent."