Riyad as-Salihin

Abu Bakrah Ath-Thaqafi (May Allah be pleased with him) reported: The Prophet (ﷺ) said: "When two Muslims are engaged in a combat against each other with their sword's and one is killed, both are doomed to Hell". I said, "O Messenger of Allah! As to the one who kills, it is understandable, but why the slain one?" He (ﷺ) replied: "He was eager to kill his opponent".

[Al-Bukhari and Muslim].