Advent Of Imam Mahdi (A.S.)
 
The Holy Prophet (S.A.W.S.) said,

"Even if a single day remains for quiyamat, Allah will send a man from my Ahlul'Bayt. He will fill the earth with justice just like it is filled with tyranny."
(Sunan Abu Dawood vol.4 Pg.107) 
Dibil Ibne Ali al-Khuzai says:
When I was reciting the Qasida (Panegeric) before my master, Raza (A.S.), I reached to the following lines.

“I hope in the advent of my Imam. Who will rise up with the name of Allah and the divine blessings will accompany him.
He will separate the truth from falsehood (for us) and he will reward and punish."
Imam (A.S.) said to me: .

"O Khuzaee ! Ruhul Qudus (the angel) has made you recite these two couplets. Do you know who this Imam is? One who will rise?”
I replied,

"No my master, I only know that one of you, Imams will rise up and clear the earth of all injustice and fill it will equity."
Imam Reza (A.S.) said,

"O Dibil after me my son Muhammad shall be the Imam and after him his son Ali, and after him his son Hasan al Askari, and after him his son Al-Qaim who shall be the awaited one during the period of his ghaibat. And when he shall be revealed, all the world shall bow down to him. Even if a single day remains for this world, Allah the High and the Mighty shall prolong this day till he will appear and fill the earth with justice as it has been filled with tyranny. And who knows when it will come to pass! I have heard it from my father, (who had heard it) from his father, and they had heard it from the Messenger of Allah (S.A.W.S.) when he was asked, "O Prophet of Allah (S.A) when will the Qaem of your progeny arise?

He (S.A.W.S.) had replied,

"His likeness is to the Hour (Quiyamat), '.....none but He shall manifest it at its time; it will be momentous in the heavens and the earth; it will not come on you but of a sudden'." 
(Sura Araf 7 : 187)
(Kamaaluddin vol.2 Pg.372-373, Faraidus Simtain vol.2. Pg.337-338. Al Fusool al Mohimma Pg. 250-251)