Riyad as-Salihin: The Book of Miscellany – Hadith No. 1226 — Abu Bakrah Ath-Thaqafi (May Allah be pleased with him) reported: The Prophet… (Hadith No. 1226)

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"..

Explanation:

Reference: Riyad as-Salihin, The Book of Miscellany, Hadith No. 1226