Exodus 19:14-18

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

He also said to the people, “Be ready for the third day; do not go near a woman.”

So it came about on the third day, when it was morning, that there were thunder and lightning flashes and a thick cloud over the mountain and a very loud trumpet sound, so that all the people who were in the camp trembled.

And Moses brought the people out of the camp to meet God, and they stood at the foot of the mountain. The Lord Visits Sinai

Now Mount Sinai was all in smoke because the Lord descended upon it in fire; and its smoke ascended like the smoke of a furnace, and the entire mountain quaked violently.