Sahih Muslim 2888 a
The Book of Tribulations and Portents of the Last Hour
If Two Muslims Confront One Another With Their Swords
Ahnaf b. Qais reported: I set out with the intention of helping this person (Hadrat 'Ali) when 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 (ﷺ), viz. 'Ali. Thereupon he said to me: Ahnaf, go back, for I heard Allah's Messenger (ﷺ) as saying: When two Muslims confront one another with swords (in hand) both the slayer and the slain would be in Fire. He (Ahnaf) said: I said, or it was said: Allah's Messenger (ﷺ) , 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.