Moses is quite the guy: super-smart and daring Hebrew mom, brave sister, educated and powerful adopted mom... and he still grows up to be a murderer. Yet, he is more because he answers God's call (albeit reluctantly and with a noble attempt to pawn it off on his brother). The poor guy gets tasked with freeing the Hebrew people from slavery, then motivating them and keeping them on track through the desert for forty years! (Though how you keep people on track while wandering seems like an ironic/futile task.)
So, while I have two quotes in a row from Moses to the people, I'll bet he actually said this a whole lot more than the scriptures recorded.
For an explanation of this Fear Not project, see my first post on fear.