Go back, for I heard the Messenger of Allah ﷺ say, "When two Muslims face each other with their swords, both the killer and the slain are in the Fire."Someone asked, "Messenger of Allah, this is the killer, but what about the one who was slain?"He replied, "He intended to kill his companion."
حدثنا أبو كامل حدثنا حماد بن زيد عن أيوب ويونس عن الحسن عن الأحنف بن قيس قال خرجت وأنا أريد يعني في القتال فلقيني
ارجع فإني سمعت رسول الله ﷺ يقول إذا تواجه المسلمان بسيفيهما فالقاتل والمقتول في النار قال يا رسول الله هذا القاتل فما بال المقتول قال إنه أراد قتل صاحبه
Reported by Muslim, chapter on Fitan, "When two Muslims confront each other with their swords," hadith 2888, from Abu Kamil, and Bukhari, chapter on Iman, "And if two groups of believers fight, then make peace between them," hadith 31, from the hadith of Hammad bin Zayd, through the same chain.