Usul al-Kafi
Book 4:
The Book of Divine Proof

Chapter 82
Reappearance of the Imam (p)

457. 2. A believer asked Imam Jafar as-Sadiq (p): Please tell me when the era which we have been impatiently waiting for will come?
The Imam (p): People who predict the date of such an occurrence are liars. They are subjecting themselves to perdition. Those who bow their heads to the Supreme Will of God and wait patiently will be emancipated.