And the Lord told Moses, "Go to the people and purify them today and tomorrow. They must wash their clothes
and be prepared by the third day, for on the third day the Lord will come down on Mount Sinai in the sight of all the people.
Put boundaries for the people all around the [mountain] and say: Be careful that you don't go up on the mountain or touch its base. Anyone who touches the mountain will be put to death.
No hand may touch him; instead he will be stoned or shot [with arrows], neither animal or man will live. When the ram's horn sounds a long blast, they may go up the mountain."
Then Moses came down from the mountain to the people and consecrated them, and they washed their clothes.
He said to the people, "Be prepared by the third day. Do not have sexual relations with women."
References for Exodus 19:15
On the third day, when morning came, there was thunder and lightning, a thick cloud on the mountain, and a loud trumpet sound, so that all the people in the camp shuddered.
Then Moses brought the people out of the camp to meet God, and they stood at the foot of the mountain.
Mount Sinai was completely enveloped in smoke because the Lord came down on it in fire. Its smoke went up like the smoke of a furnace, and the whole mountain shook violently.
As the sound of the trumpet grew louder and louder, Moses spoke and God answered him in the thunder.
The Lord came down on Mount Sinai, at the top of the mountain. Then the Lord summoned Moses to the top of the mountain, and he went up.
The Lord directed Moses, "Go down and warn the people not to break through to see the Lord; otherwise many of them will die.
Even the priests who come near the Lord must purify themselves or the Lord will break out [in anger] against them."
But Moses responded to the Lord, "The people cannot come up Mount Sinai, since You warned us: Put a boundary around the mountain and consider it holy."