Ahnaf bin Qays reported,
I set out to support this man, but Abu Bakra met me and asked, "Where are you headed?"I replied, "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 against each other, 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 slain?'He answered, 'He too was eager to kill his companion.'"
Azami 6794
Sahihعن الأحنف بن قيس قال
ذهبت لأنصر هذا الرجل 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 through the hadith of Hammad bin Zayd, who said, "Ayub and Yunus narrated to us, from Hasan, from Ahnaf bin Qays," and he mentioned it. Muslim also narrated it through the hadith of Muhammad bin Jafar Ghundar, from Shuba, from Mansur, from Rabi bin Harash, from Abu Bakra, from the Prophet ﷺ, "When two Muslims each raise a weapon against the other, they are both on the brink of Hell, and if one of them kills the other, they both enter it together."However, Bukhari (7083) said, "Ghundar said, Shuba narrated to us, from Mansur, from Rabi bin Harash, from Abu Bakra, from the Prophet ﷺ," and Sufyan did not narrate it from Mansur as a raised hadith.Nawawi, may Allah have mercy on him, says in his commentary on Muslim, "Know that the bloodshed that occurred among the companions, 
is not included in this threat. The doctrine of the people of the sunnah and truth is to think well of them, to refrain from what occurred among them, and to interpret their fighting as the result of their exercising independent judgment."