Holy Prophet (s.a.w.s.) said, "My Caliphs are my legatees and the proofs of Allah upon the creatures, after me they are twelve. The first of them is my brother and the last of them is my (grand) son"
He was asked, "0 Prophet of Allah, which brother of yours"
He replied, "Ali Ibn Abu Talib"
Again he was asked, "And which Son?"
"Al Mahdi is the one who will fill the earth with justice and equity just like it had been filled with injustice and tyranny. I swear by the one who has sent me as the giver of good tidings, even is a single day remains for the world (to end), Allah shall prolong that day till he sends (to it) my son Mahdi. Then Ruhullah (spirit) Isa ibne Maryam will descend and perform prayers behind him. The earth shall be illuminated by his radiance and his authority shall stretch from the east to the west."
(Faraidus Simtam vol.2 Pg.312)