Narrated Al-Ahnaf bin Qais:
While I was going to help this man ('Ali Ibn Abi Talib), Abu Bakra met me and asked, "Where are you going?" I replied, "I am going to help that person." He said, "Go back for I have heard Allah's Messenger (ﷺ) saying, 'When two Muslims fight (meet) each other with their swords, both the murderer as well as the murdered will go to the Hell-fire.' I said, 'O Allah's Messenger! It is all right for the murderer but what about the murdered one?' Allah's Messenger (ﷺ) replied, "He surely had the intention to kill his companion."
Narrated Al-Ahnaf bin Qais:
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! It is alright for the killer, but what about the killed one?' He said, 'The killed one was eager to kill his opponent."
Narrated Al-Hasan:
(Al-Ahnaf said:) I went out carrying my arms during the nights of the affliction (i.e. the war between `Ali and `Aisha) and Abu Bakra met me and asked, "Where are you going?" I replied, "I intend to help the cousin of Allah's Messenger (i.e.,`Ali)." Abu Bakra said, "Allah's Messenger (ﷺ) said, 'If two Muslims take out their swords to fight each other, then both of them will be from amongst the people of the Hell- Fire.' It was said to the Prophet, 'It is alright for the killer but what about the killed one?' He replied, 'The killed one had the intention to kill his opponent.'" (See Hadith No. 30, Vol. 1)
Narrated Al-Ahnaf:
Abu Bakra said: The Prophet (ﷺ) said (as above, 204).
Narrated `Abdullah bin `Umar:
Allah's Messenger (ﷺ) said, "A Muslim is a brother of another Muslim, so he should not oppress him, nor should he hand him over to an oppressor. Whoever fulfilled the needs of his brother, Allah will fulfill his needs; whoever brought his (Muslim) brother out of a discomfort, Allah will bring him out of the discomforts of the Day of Resurrection, and whoever screened a Muslim, Allah will screen him on the Day of Resurrection . "