Al-Ahnaf bin Qays (may Allah have mercy on him) said: I set out intending to help a man, and Abu Bakrah (may Allah be pleased with him) met me and asked: Where are you heading? I said: To help this man. He said: Go back, for I heard the Prophet (peace and blessings be upon him) say: "When two Muslims fight each other with their swords, both the killer and the slain are in Hell." I said: O Messenger of Allah, the killer (deserves it), but what is the fault of the slain? He (peace and blessings be upon him) said: "He too was eager to kill his companion."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16793