Sayyiduna Abu Bakrah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "When Muslims take up swords and one of them kills the other, then both the killer and the slain are people of Hell."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16792