Sahih Muslim
The Book Pertaining to the Turmoil and Portents of the Last Hour (Kitab Al-Fitan wa Ashrat As-Saah)
Book Number: 041 / Hadith Number: 6899
Ahnaf b. Qais reported on the authority of Abu Bakra that Allah's Messenger (may peace be upon him) said: When two Muslims confront each other with their swords, both the slayer and the slain are doomed to Hell-Fire.