Imam al-Shafi'i (may Allah have mercy on him) said: "Allah Almighty called them (the two groups fighting each other) believers, and commanded that peace be made between them."
Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "The Hour will not be established until two great groups fight each other. There will be a great slaughter between them, and their claim will be one."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16708