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

Chapter 14
The pillars on the earth

335. 3. Imam Muhammad al-Baqir (p): The eminence of Imam Ali (p) is that he accepted what was brought by Prophet Muhammad (p), and refrained from what he forbade. Obligation to obey him (Imam Ali (p)) was established after the demise of the Prophet Muhammad (p) in the same way as that for the Prophet Muhammad (p), but superiority belongs to the Prophet Muhammad (p). One who projects himself ahead of Imam Ali (p) is as though he wished to project himself ahead of God and His Prophet. And one who imagines himself to be superior to Imam Ali (p) is as if he imagined himself to be superior to the Prophet Muhammad (p). And one, who rejects any of the rulings of Imam Ali (p), small or big, reflects polytheism.
Prophet Muhammad (p) is the door to God. No one reaches God unless he passes through this door. The Prophet Muhammad (p) is the path to God. And whoever took this path found God, to Whom belong Might and Majesty. And after the Prophet Muhammad (p), Imam Ali (p) is (of) the same (degree), after him the Imams (p); one after the other. God has made them the pillars of the earth who kept it stable with all its inhabitants. God has made the Imams the mainstay of Islam and the keepers of the path of true guidance to Him. No one is guided except through their guidance and no one will go astray except through his negligence in fulfilling his obligations towards the Imams (p). The Imams (p) are the custodians of the knowledge of God, the bounties, and the warnings that God has sent down. The Imams (p) are the established Proof for whoever is on the earth. All that was established for the first of them (Imams (p)) is established by God for the last of them. And no one can reach this status except with the help of God.