Exodus 19:16 (KJV)

And Moses went down from the mount unto the people, and sanctified the people; and they washed their clothes.

And he said unto the people, Be ready against the third day: come not at your wives.

And it came to pass on the third day in the morning, that there were thunders and lightnings, and a thick cloud upon the mount, and the voice of the trumpet exceeding loud; so that all the people that was in the camp trembled.

And Moses brought forth the people out of the camp to meet with God; and they stood at the nether part of the mount.

And mount Sinai was altogether on a smoke, because the Lord descended upon it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly.