Then the LORD said to Moses, "Go to the people and consecrate them today and tomorrow, and let them wash their clothes.
Exodus 19:10
Surrounding context
19:9 And the LORD said to Moses, "Behold, I come to you in the thick cloud, that the people may hear when I speak with you, and believe you forever." So Moses told the words of the people to the LORD.
19:10 Then the LORD said to Moses, "Go to the people and consecrate them today and tomorrow, and let them wash their clothes.
19:11 And let them be ready for the third day. For on the third day the LORD will come down upon Mount Sinai in the sight of all the people.