Exodus 19:16-19

16And it came to pass on the third day in the morning, that there were thunders and lightnings, and a thick cloud upon the mount, and the voice of the trumpet exceeding loud; so that all the people that were in the camp trembled. 17And Moses brought forth the people out of the camp to meet with God; and they stood at the lower part of the mount. 18And mount Sinai was altogether in a smoke, because the LORD descended upon it in fire: and the smoke of it ascended as the smoke of a furnace, and the whole mount trembled greatly. 19And when the voice of the trumpet sounded long, and grew louder and louder, Moses spoke, and God answered him by a voice.