Exodus 19:16-18

Then it came to pass on the third day, in the morning, that there were thunderings and lightnings, and a thick cloud on the mountain; and the sound of the trumpet was very loud, so that all the people who were in the camp trembled. And Moses brought the people out of the camp to meet with God, and they stood at the foot of the mountain. Now Mount Sinai was completely in smoke, because the LORD descended upon it in fire. Its smoke ascended like the smoke of a furnace, and the whole mountain quaked greatly.