Even if the entire duration of the world's existence has already been exhausted and only one day is left before Doomsday (Day of judgment), Allah will expand that day to such a length of time, as to accommodate the kingdom of a person out of my Ahlul-Bayt who is called by my name. He will then fill out the earth with peace and justice as it will have been full of injustice and tyranny before then. Sahih Tirmidhi along with 24 other Sunni sources
This narrative is very similar to those of Shiites but Sunnis think that Imam Mahdi (AS) is not alive and it may not be born yet. Shia on the other hand says that Mahdi (AS) is alive and he is son of Imam Hassan Askari (AS) the 11th Imam of Shiites. This confirms the above narrative that emphasizes Imam Mahdi is from Ahlul-Bayt. It is also compatible with the Hadith that tells: If someone die and don't recognize his Imam he is died in the way of Jaheliyah (Era before the Prophet (SAWA) ). To know our Imams it must be alive.