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.
Exodus 19:18
Surrounding context
19:17 And Moses brought the people out of the camp to meet with God, and they stood at the foot of the mountain.
19:18 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.
19:19 And when the blast of the trumpet sounded long and became louder and louder, Moses spoke, and God answered him by voice.