Imam Mahdi (A.S.) in Sunni Book

The Twelve Guides

In a tradition the Holy Prophet said to the Commander of believers, Ali, that: 
"There will be twelve Guides (Imams) after me, the first of whom is you, O' Ali, and the last one will be the 'Support' (al-Qa'im), who with the grace of Allah, will gain victory over the whole east and west of the world."

Imam Mahdi (a.s.) in Sunni Books

Hadhrat Ali (radhiyallahu anhu) narrates that Rasulullah sallallahu alayhi wasallam  said, 

"Even if only a day remains for Qiyamah to come, yet Allah will surely send a man from my family who will fill this world with such justice and fairness, just as it initally was filled with oppression."

(Abu Daawood)

Ziyarat of Aale Yaaseen

Denying the Coming of Imam Mahdi (a.s.)

Holy Prophet of Islam (s.a.) Said:
One who denies the arising of Mahdi, then indeed, he has disbelieved.