Hebrews 12:21

And so fearful was the appearance, that Moses said, I exceedingly fear and quake.

The fact of Moses' being afraid is mentioned in Deuteronomy 9:19; but the exact quotations given by the author here could well have come to him from some other source, or perhaps from his own independent knowledge of it by reason of his inspiration. The emphasis is that the most frightening things taking place on Sinai were so utterly terrifying that godly Moses, despite his royal education, was also exceedingly afraid and trembled at the sight of it. This makes it clear that it was not the unreasonable and ignorant fear of a nation of slaves which came at times, but a truly justified fear of the majesty and power of God.