Sunan Ibn Majah, Chapter: The Chapters on Tribulations

that the Messenger of Allah (saw) said: that the Messenger of Allah (saw) said:"Three will fight one another for your treasure, each one of them the son of a caliph, but none of them will gain it. Then the black banners will come from the east, and they will kill you in an unprecedented manner." Then he mentioned something that I do not remember, then he said: "When you see them, then pledge your allegiance to them even if you have to crawl over the snow, for that is the caliph of Allah, Mahdi."