The Messenger of Allah ﷺ said, "When two Muslims confront each other with their swords, both the killer and the slain are in the Fire."The companions asked, "Messenger of Allah, this is the killer, but what about the slain?"He replied, "He too intended to kill his companion."
حدثنا أحمد بن سنان حدثنا يزيد بن هارون عن سليمان التيمي وسعيد بن أبي عروبة عن قتادة عن الحسن
قال رسول الله ﷺ إذا التقى المسلمان بسيفيهما فالقاتل والمقتول في النار قالوا يا رسول الله هذا القاتل فما بال المقتول قال إنه أراد قتل صاحبه
[Sahih] Reported by Nasai, 7/124, Prohibition of Blood, Prohibition of Killing, no. 4123, 4124, and Ibn Abu Aruba did not mention it, and Yunus reported it from Hasan on the same authority, Nasai, 7/126, from the hadith of Yazid, with the same wording, no. 4129. The hadith has corroborating narrations, including the one that follows, and the hadith of Ibn Majah, which Busiri graded sahih.