The Messenger of Allah ﷺ said, "When two Muslims confront each other with their swords and one kills the other, both the killer and the slain are in the Fire."Someone asked, "This is the killer, but what about the one who was killed?"He replied, "He too had intended to kill his companion."
حدثنا مؤمل بن إسماعيل حدثنا حماد بن زيد حدثنا المعلى بن زياد ويونس وأيوب وهشام عن الحسن عن الأحنف
قال رسول الله ﷺ إذا تواجه المسلمان بسيفيهما فقتل أحدهما صاحبه فالقاتل والمقتول في النار قيل هذا القاتل فما بال المقتول قال قد أراد قتل صاحبه
Its isnad is sahih. Mualla bin Ziyad is al-Qurdusi, and he is trustworthy, his hadith is found in Muslim. The rest of the narrators are trustworthy and well known. The hadith has already appeared at 20303 and 19639.