Sahih Muslim Book 41, Hadith Number 6898.

Chapter : When two Muslims confront each other with swords.

Ahnaf b. Qais reported: I set out with the intention of helping this person (Hadrat ‘Ali) that Abu Bakra met me. He said: Ahnaf, where do you intend to go? I said: I intend to help the cousin of Allah’s Messenger (may peace be upon him), viz. ‘Ali. Thereupon he said to me: Ahnaf, go back, for I heard Allah’s Messenger (may peace be upon him) as saying: When two Muslims confront one another with swords (in hands) both the slayer and the slain would be in Fire. He (Ahnaf) said: I said, or it was said: Allah’s Messenger, it may be the case of one who kills. but what about the slain (why he would be put in Hell-Fire)? Thereupon he said: He also intended to kill his companion.