The Book of Tribulations and Portents of the Last Hour
كتاب الفتن وأشراط الساعة
(4) Chapter: If Two Muslims Confront One Another With Their Swords
باب إِذَا تَوَاجَهَ الْمُسْلِمَانِ بِسَيْفَيْهِمَا
Abu Bakra reported Allah’s Messenger (ﷺ) as saying:
When two Muslims (confront each other) and the one amongst them attacks his brother with a weapon, both of them are at the brink of Hell-Fire. And when one of them kills his companion, both of them get into Hell-Fire.