(4) باب إِذَا تَوَاجَهَ الْمُسْلِمَانِ بِسَيْفَيْهِمَا
Ahnaf b. Qais reported on the authority of Abu Bakra that Allah's Messenger (ﷺ) said:
When two Muslims confront each other with their swords, both the slayer and the slain are doomed to Hell-Fire.
Reference: Sahih Muslim 2888 b
In-book reference: Book 54, Hadith 19
USC-MSA web (English) reference(deprecated numbering scheme): Book 41, Hadith 6899