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

Chapter 52
Modesty

781. 2. Imam Jafar as-Sadiq (p): Modesty, dignified social life, abstinence from useless prattle and leniency of heart are among the ingredients of belief.(B5,C52,T2)

782. 4. Imam Muhammad al-Baqir (p): Modesty and belief are interlinked. If one goes away, the other follows it.(B5,C52,T4)

783. 5. Imam Jafar as-Sadiq (p): One who has no modesty has no belief.(B5,C52,T5)