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

Chapter 16
Islam precedes belief

634. 2. Samaa requested Imam Jafar as-Sadiq (p) to explain the difference between ‘Islam’ and ‘Belief’ (Iman).
The Imam (p): I shall give you an example. ‘Islam’ and ‘Belief’ are like the Sacred Precincts and the Kaba respectively. Sometimes a person is inside the Holy Precincts but not inside the Kaba and also cannot reach inside the Kaba without entering into the Sacred Precincts. Similarly, a person can be a Muslim but not a believer (Momin). But he cannot be a believer (Momin) unless he is a Muslim…(B5,C16,T2)