Exodus 19:9-11

And the LORD said to Moses, “Behold, I come to you in the thick cloud, that the people may hear when I speak with you, and believe you forever.” So Moses told the words of the people to the LORD. Then the LORD said to Moses, “Go to the people and consecrate them today and tomorrow, and let them wash their clothes. And let them be ready for the third day. For on the third day the LORD will come down upon Mount Sinai in the sight of all the people.