Chapter 37: Trials and Fierce Battles (Kitab Al-Fitan Wa Al-Malahim)
Ahnaf b. Qais said: I came out with the intention of (participating in) fighting. Abu Bakrah met me and said: Go back, for I heard the Messenger of Allah (ﷺ) say: When two Muslims face each other with their swords, the killer and the slain will go to Hell. He asked: Messenger of Allah, this is the killer (so naturally he should go to Hell), but what is the matter with the slain ? He replied: He intended to kill his companion.
Sunan Abi Dawud 4268