It is narrated from Hazrat Abu Musa that the Prophet (peace be upon him) said: When two Muslims come face to face with swords drawn and one of them kills the other, then both the killer and the killed will go to Hell.
Hadith Referenceمسند احمد / أول مسند البصريين / 20519
Hadith Gradingحکم دارالسلام:حديث صحيح، خ: 31، م: 2888، مؤمل سي الحفظ لكنه توبع