Albert Barnes Commentary Exodus 20:18-21

"And all the people perceived the thunderings, and the lightnings, and the voice of the trumpet, and the mountain smoking: and when the people saw it, they trembled, and stood afar off. And they said unto Moses, Speak thou with us, and we will hear; but let not God speak with us, lest we die. And Moses said unto the people, Fear not: for God is come to prove you, and that his fear may be before you, that ye sin not. And the people stood afar off, and Moses drew near unto the thick darkness where God was." — Exodus 20:18-21 (ASV)

Compare with Deuteronomy 5:22-31. Aaron (Exodus 19:24) on this occasion accompanied Moses in drawing near to the thick darkness.

Exodus 20:22 to Exodus 23:33 is a series of laws which we may identify with what was written by Moses in the book called the book of the covenant, and read by him in the hearing of the people (Exodus 24:7).

The document cannot be regarded as a strictly systematic whole. Portions of it were probably traditional rules handed down from the patriarchs and retained by the Israelites in Egypt.