I set out to support this man, meaning Ali bin Abu Talib, when Abu Bakra met me and asked, "Where are you headed?"I said, "I am going to support this man."He said, "Go back, for I heard the Messenger of Allah ﷺ say, 'When two Muslims meet with their swords drawn, both the killer and the slain are in the Fire.' I said, 'Messenger of Allah, this is the killer, but what about the slain?' He replied, 'He was determined to kill his companion.'"
ذهبت لأنصر هذا الرجل ( يعني علي ابن أبي طالب) فلقيني أبو بكرة فقال: أين تريد؟ قلت: أنصر هذا الرجل. قال: ارجع، فإني سمعت رسول الله ﷺ يقول: إذا التقى المسلمان بسيفيهما فالقاتل والمقتول في النار. فقلت: يا رسول الله، هذا القاتل، فما بال المقتول؟ قال: إنه كان حريصا على قتل صاحبه.
Bukhari narrated it in the Book of Faith (31), and Muslim in the Book of Tribulations (2888), both from the hadith of Hammad bin Zayd, from Ayub and Yunus, from Hasan, from Ahnaf bin Qays, who mentioned it.