Exodus 19:20-22

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.