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