Exodus 19:17-19

17Then Moses brought the people out of the camp to meet God, and they stood and presented themselves at the foot of the mountain.

18Mount Sinai was wrapped in smoke because the Lord descended upon it in fire; its smoke ascended like the smoke of a furnace, and the whole mountain quaked violently.

19And it happened, as the blast of the ram’s horn grew louder and louder, Moses spoke and God answered him with [a voice of] thunder.