Riyad as-Salihin, Book 1, Hadith 9 (In-Book) ⓘ

Abu Bakrah Ath-Thaqafi (May Allah be pleased with him) reported: The Prophet 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 replied: "He was eager to kill his opponent".[Al-Bukhari and Muslim].

وعن أبي بكرة نفيع بن الحارث الثقفى رضي الله عنه أن النبي صلى الله عليه وسلم قال‏:‏ ‏"‏إذ التقى المسلمان بسيفيهما فالقاتل والمقتول في النار‏"‏ قلت يارسول الله، هذا القاتل فما بال المقتول‏؟‏ قال‏:‏ ‏"‏إنه كان حريصاً على قتل صاحبه‏"‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏‏.‏