Usul al-Kafi
Book 5:
The Book of Belief and Disbelief

Chapter 18
Parts of body and belief

638. 1. Imam Jafar as-Sadiq (p) was asked: Which is the best deed in the eye of God without which no other good deed is accepted?
The Imam (p): To believe in God and to acknowledge His Oneness. This is the greatest and best deed and highest in status and degree.
Imam Jafar as-Sadiq (p) was further asked: Is belief a combination of declaration and action, or mere declaration without action?
The Imam (p): Belief is action and declaration is a part of this action, enjoined by God and recorded in His Book. His glory is manifest and His existence has been proved. His Book is a witness to this and invites people to it (belief with action)…(B5,C18,T1)

639. 2. Imam Jafar as-Sadiq (p): Without doubt, every part of the body will be questioned for what it has done. Ears will be questioned as to what they have heard, eyes as to what they have seen and the heart (mind) will be questioned regarding its faith and belief.(B5,C18,T2)

640. 4. Imam Jafar as-Sadiq (p) was asked: What exactly is Islam?
The Imam (p): The religion approved by God is Islam. This religion existed before you and will continue to exist even after you. One who professes that Islam is the approved religion of God is a Muslim and one who obeys the commandments of God is a believer.(B5,C18,T4)

641. 6. Imam Jafar as-Sadiq (p) was requested to explain ‘Belief.’
The Imam (p): It is to bear witness that there is no god but God and that Prophet Muhammad (p) is His Prophet. Action according to declaration of belief is an essential element of belief. Belief cannot be proved without matching action.(B5,C18,T6)