It was narrated from Thawban that the Messenger of Allah (ﷺ) 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."