At a time when Imam Hassan (AS) and Imam Hussain (AS) were present, Imam Ali (AS) told people:

O’ people, beware of your time, swear to God, my two sons and I will be martyred. Then, with God’s permission at the end, one of my children will appear to avenge our blood; he will not be seen by people in order that the corrupt people are unable to recognize him, and this occultation will be so long that the ignorant people will say: God has no plan for the family of Mohammad (PBUH) [1]. Mofazzal narrated from Imam Sadeq (AS) who said: One day, Imam Ali (AS) was preaching in Kufah Mosque and told people: O’ people, beware that the earth will never be without the proof of God (infallible Imam). But Allah (SWT) will deprive people from seeing the proof of God because of their oppression, cruelty and extravagance which they do to themselves (meaning, for their actions in future, people will be deprived of seeing infallible Imam) [2].

[1] Qaybat-e-No’amani, Page Page 140.

[2] Beharul Anwaar, Vol. 51, Page 112.

Source: en.shafaqna.com