Exodus 19:10-12, 14

The LORD also said to Moses, “Go to the people and consecrate them today and tomorrow, and let them wash their garments; and let them 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. You shall set bounds for the people all around, saying, ‘Beware that you do not go up on the mountain or touch the border of it; whoever touches the mountain shall surely be put to death.

So Moses went down from the mountain to the people and consecrated the people, and they washed their garments.