Exodus 19:17-19

19:17 Moses brought the people out of the camp to meet God, and they took their place at the foot of the mountain. 19:18 Now 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. 19:19 When the sound of the horn grew louder and louder, Moses was speaking and God was answering him with a voice.