Imaam Mahdi will certainly come before the day of Qiyaamat. Rasulullah Sall-Allahu alayhi wa sallam said, ‘Even if there were only one day left for the world, Allah would send a man from amongst us to fill the world with justice, just as it had been filled with oppression and injustice.’ (Ahmad)
Rasulullah Sall-Allahu alayhi wa sallam said, ‘The Mahdi is one of us, from among the people of my household. In one night Allah will inspire him and prepare him to carry out his task successfully.’ (Ahmad)
Abu Dawud devoted a whole chapter of his Sunan on the subject of the Mahdi.
Abdullah, Radi-Allahu anhu, narrates that Rasulullah Sall-Allahu alayhi wa sallam said, ‘A man from my household whose name is like mine will take power.’ (Tirmidhi)
Thawbaan narrates that Rasulullah Sall-Allahu alayhi wa sallam said, ‘Three men will be killed at the place where your treasure is. Each of them will be the son of a Khalifah and none of them will get hold of the treasure. Then the black banners will come out of the east, and they will slaughter you in a way which has never been seen before.’ Then he said something which I do not remember, ‘then if you see him, go and give him your allegiance, even if you have to crawl over ice because he is the Khalifah of Allah, the Mahdi.’ (Ibn Majah) The treasure referred to in this text is the treasure of the Ka’bah. Towards the end of time, three of the sons of the Khalifahs will fight to get hold of it, until the Mahdi appears.
Abu Hurayra, Radi-Allahu anhu, narrates that Rasulullah Sall-Allahu alayhi wa sallam said, ‘By him in whose hand my soul is, surely the son of Mary will come down among you as a just ruler. He will break the cross, kill the pigs and abolish the Jizyah. Wealth will be in such abundance that no one will care about it, and a single prostration in prayer will be better than the world and all that is in it.’ Abu Hurayra said, ‘If you wish recite the verse - and there is none of the people of the book but must believe in him before his death, and on the day of Qiyaamat he will be a witness against them.’ (An-Nisaa 4:159) (Bukhari; Muslim). And Allah Ta’ala Knows Best.