He said, I heard the Messenger of Allah ﷺ say, "When two Muslims confront each other with their swords and one kills the other, both the killer and the slain are in the Fire."They asked, "Messenger of Allah, this is the killer, but what about the one who was killed?"He replied, "He too was intent on killing his companion."
أخبرنا أحمد بن فضالة قال حدثنا عبد الرزاق قال أنبأنا معمر عن أيوب عن الحسن عن الأحنف بن قيس
قال سمعت رسول الله ﷺ يقول إذا تواجه المسلمان بسيفيهما فقتل أحدهما صاحبه فالقاتل والمقتول في النار قالوا يا رسول الله هذا القاتل فما بال المقتول قال إنه أراد قتل صاحبه
Bukhari transmitted it in the Book of Tribulations, chapter "When Two Muslims Meet with Their Swords," hadith no. 7083, from the hadith of Muamar bin Rashid, as a mualaq narration. Muslim transmitted it in the Book of Tribulations, chapter "When Two Muslims Confront Each Other with Their Swords," hadith no. 2888, from the hadith of Ayub Sakhtiyani, with the same wording. It is also found in Kubra, hadith no. 3587.