Exodus 19:9-11

And Jehovah said unto Moses, Lo, I come unto thee in a thick cloud, that the people may hear when I speak with thee, and may also believe thee for ever. And Moses told the words of the people unto Jehovah. And Jehovah said unto Moses, Go unto the people, and sanctify them to-day and to-morrow, and let them wash their garments, and be ready against the third day; for the third day Jehovah will come down in the sight of all the people upon mount Sinai.