Exodus 19:18-22

And mount Sinai was altogether on a smoke, because the LORD descended upon it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly.

And when the voice of the trumpet sounded long, and waxed louder and louder, Moses spake, and God answered him by a voice.

And the LORD came down upon mount Sinai, on the top of the mount: and the LORD called Moses up to the top of the mount; and Moses went up.

And the LORD said unto Moses, Go down, charge the people, lest they break through unto the LORD to gaze, and many of them perish.

And let the priests also, which come near to the LORD, sanctify themselves, lest the LORD break forth upon them.