Exodus 19:9-11

The LORD said to Moses, “Behold, I will come to you in a thick cloud, so that the people may hear when I speak with you and may believe and trust in you forever.” Then Moses repeated the words of the people to the LORD. The LORD also said to Moses, “Go to the people and consecrate them today and tomorrow [that is, prepare them for My sacred purpose], and have them wash their clothes and be ready by the third day, because on the third day the LORD will come down on Mount Sinai [in the cloud] in the sight of all the people.