Exodus 19:9-11 ESV
And the LORD said to Moses, “Behold, I am coming to you in a thick cloud, that the people may hear when I speak with you, and may also believe you forever.”
When Moses told the words of the people to the LORD, the LORD said to Moses, “Go to the people and consecrate them today and tomorrow, and let them wash their garments and be ready for the third day. For on the third day the LORD will come down on Mount Sinai in the sight of all the people.
ESV: English Standard Version 2016