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

Chapter 5
There can be no world without Hujjat

303. 1. Imam Jafar as-Sadiq (p) was asked by a man: Can the world ever be devoid of Hujjat? The Imam (p): No.
The Man: Can there be two Imams at a time?
The Imam (p): No, if two Imams are living at the same time, only one will act as an Imam.

304. 4. Imam Jafar as-Sadiq (p): The world cannot exist without a divinely guided Imam (Hujjat of God).

305. 5. Imam Jafar as-Sadiq (p): God has never left this world without someone possessing perfect knowledge (an Imam); otherwise distinction between right and wrong would not be possible.
306. 8. Imam Muhammad al-Baqir (p): Since the death of Prophet Adam (p), God has not left the earth without an Imam who guides people towards the path of God. This Imam is the Hujjat of God for His servants and the earth cannot exist without an Imam.