Exodus 19:21

18 And all the mountain of Sinai was smoking, for the Lord had come down on it in fire: and the smoke of it went up like the smoke of a great burning; and all the mountain was shaking.

19 And when the sound of the horn became louder and louder, Moses' words were answered by the voice of God.

20 Then the Lord came down on to Mount Sinai, to the top of the mountain, and the Lord sent for Moses to come up to the top of the mountain, and Moses went up.

21 And the Lord said to Moses, Go down and give the people orders to keep back, for fear that a great number of them, forcing their way through to see the Lord, may come to destruction.

22 And let the priests who come near to the Lord make themselves holy, for fear that the Lord may come on them suddenly.

23 And Moses said to the Lord, The people will not be able to come up the mountain, for you gave us orders to put limits round the mountain, marking it out and making it holy.

24 And the Lord said to him, Go down, and you and Aaron may come up; but let not the priests and the people make their way through to the Lord, or he will come on them suddenly.