Exodus 19:20-22

20The Lord came down onto the top of Sinai mountain. Then the Lord told Moses that he must come up to the top of the mountain. So Moses went up the mountain.

21The Lord said to Moses, ‘Go down to the people. Warn them that they must stay away from the mountain. They must not cross the line to try to see me, the Lord. If they do that, many of them will die.

22Even the priests must make themselves clean before they come near to me. If they do not obey this command, I will punish them. I am the Lord.’