The Prophet ﷺ said, "When two Muslims confront each other with their swords and one kills the other, both are in the Fire."Someone asked, "Messenger of Allah, this is the killer, but what about the one who was killed?"He replied, "He intended to kill his companion."
أخبرني محمد بن إسمعيل بن إبراهيم عن يزيد عن سليمان التيمي عن الحسن
عن النبي ﷺ قال إذا تواجه المسلمان بسيفيهما فقتل أحدهما صاحبه فهما في النار قيل يا رسول الله هذا القاتل فما بال المقتول قال أراد قتل صاحبه
[Sahih] Ibn Majah reported it in Fitan, chapter, "When two Muslims meet with their swords," no. 3964, from the hadith of Yazid bin Harun with this same chain. It is also in Kubra, no. 3583. See the following hadith.