Messenger of Allah (Sallallaahu ‘alaihi wasallam) passed by a man of the Ansar who was admonishing his brother regarding shyness. Messenger of Allah (Sallallaahu ‘alaihi wasallam) said, “Leave him alone, for modesty is a part of Iman.”
[Al-Bukhari and Muslim]
The Ansari, who was being lectured by his brother, was indeed an embodiment of modesty and unpretentiousness. Such a man is never sharp in worldly affairs because modesty restrains him from following wrong ways. For this reason, modesty has been called a part of the faith. This is an inborn quality, that is, some people are unassertive by birth. Yet, if they are gradually guided and directed towards virtues, their unassumingness further increases. And in Islam it is counted a virtue.
Riyad-us-Saliheen – Hadith No : 681