Sahih al-Bukhari Book 87, Hadith 14
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."
The Prophet (ﷺ) said, "Whoever carries arms against us, is not from us."
A Jew crushed the head of a girl between two stones, and the girl was asked, "Who has done that to you, so-and-so or so and so?" (Some names were mentioned for her) till the name of that Jew was mentioned (whereupon she agreed). The Jew was brought to the Prophet (ﷺ) and the Prophet (ﷺ) kept on questioning him till he confessed, whereupon his head was crushed with stones.