Sunan Abi Dawud — Trials and Fierce Battles (Kitab Al-Fitan Wa Al-Malahim) — Hadith No: 4268
Translation
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