Exodus 19:17-19

17And Moses brought the people out of the camp to meet with God, and they stood at the foot of the mountain.

18Now 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.

19And when the blast of the trumpet sounded long and became louder and louder, Moses spoke, and God answered him by voice.