Exodus 19:17-19

17Moses brought the people out of the camp to meet God, and they took their place at the foot of the mountain.

18Now Mount Sinai was completely covered with smoke because the Lord had descended on it in fire, and its smoke went up like the smoke of a great furnace, and the whole mountain shook violently.

19When the sound of the horn grew louder and louder, Moses was speaking and God was answering him with a voice.