Exodus 19:9

And the Lord said unto Moses, Lo, I come unto thee in a thick cloud, whose darkness would, in a measure, conceal His glory, that the people may hear when I speak with thee, and believe thee for ever. The position of Moses as the receiver and the mediator of the divine revelation would thus be attested, and the revelation and the authority of the Law, as given by him, would be established. The fact that he teaches the Word of the Lord gives to every true preacher his authority to this day. And Moses told the words of the people unto the Lord.