Riyad as-Salihin 9
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] .
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)‏‏)‏‏.‏