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

Chapter 10
Belief is tranquillity of heart

617. 1. Imam Muhammad al-Baqir (p) was requested to comment on the verse: He it is who sent down tranquillity into the hearts of the believers … (Q, 48:4) and the verse: … And strengthened them with a spirit from Himself (Q, 58:22).
The Imam (p): It means faith (Iman).(B5,C10,T1)

618. 5. Imam Jafar as-Sadiq (p) was requested to comment on the verse: He it is who sent down tranquillity into the hearts of the believers … (Q, 48:4), on the verse: … And strengthened them with a spirit from Himself (Q, 58:22), and on the verse: … And made them keep the word of observance of duty … (Q, 48:26).
The Imam (p): ‘Tranquillity’, ‘Spirit’ and ‘observance of duty’ all signify faith.(B5,C11,T5)