Exodus 19:21 (KJV)

And 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 was in the camp trembled.

And Moses brought forth the people out of the camp to meet with God; and they stood at the nether part of the mount.

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

And Moses said unto the LORD, The people cannot come up to mount Sinai: for thou chargedst us, saying, Set bounds about the mount, and sanctify it.

And the LORD said unto him, Away, get thee down, and thou shalt come up, thou, and Aaron with thee: but let not the priests and the people break through to come up unto the LORD, lest he break forth upon them.

So Moses went down unto the people, and spake unto them.