Exodus 19:9-11

Amplified Bible

⁹ 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.

¹⁰ 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 ¹¹ 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.