Exodus 19:9-11

19:9 The Lord said to Moses, “I am going to come to you in a dense cloud, so that the people may hear when I speak with you and so that they will always believe in you.” And Moses told the words of the people to the Lord.

19:10 The Lord said to Moses, “Go to the people and sanctify them today and tomorrow, and make them wash their clothes 19:11 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.