I went out to support this man, meaning Ali bin Abu Talib, when Abu Bakra met me and asked, "Where are you headed?"I replied, "I am going to support this man."He said, "Turn back, for I heard the Messenger of Allah ﷺ say, 'When two Muslims confront each other with their swords, both the killer and the slain are in the Fire.' I asked, 'Messenger of Allah, this is the killer, but what about the one who was killed?' He answered, 'He was eager 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 بسيفهما 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 of them from the hadith of Hammad bin Zayd, from Ayub and Yunus, from Hasan, from Ahnaf bin Qays, and so he mentioned it.A note, this hadith is missing from the narration of Abu Dharr Harawi, and for that reason the Hafiz did not explain it in Fath al-Bari in the Book of Faith. Rather, mention of it comes in the Book of Blood Money (6875), and he explained it there.