Exodus 19:7-25 (NRSV)
Moses meets God on the mountain
So Moses came, summoned the elders of the people, and set before them all these words that the Lord had commanded him. The people all answered as one: "Everything that the Lord has spoken we will do." Moses reported the words of the people to the Lord. Then the Lord said to Moses, "I am going to come to you in a dense cloud, in order that the people may hear when I speak with you and so trust you ever after."
When Moses had told the words of the people to the Lord, the Lord said to Moses: "Go to the people and consecrate them today and tomorrow. Have them wash their clothes and prepare for the third day, because on the third day the Lord will come down upon Mount Sinai in the sight of all the people. You shall set limits for the people all around, saying, 'Be careful not to go up the mountain or to touch the edge of it. Any who touch the mountain shall be put to death. No hand shall touch them, but they shall be stoned or shot with arrows; whether animal or human being, they shall not live.' When the trumpet sounds a long blast, they may go up on the mountain." So Moses went down from the mountain to the people. He consecrated the people, and they washed their clothes. And he said to the people, "Prepare for the third day; do not go near a woman."
On the morning of the third day there was thunder and lightning, as well as a thick cloud on the mountain, and a blast of a trumpet so loud that all the people who were in the camp trembled. Moses brought the people out of the camp to meet God. They took their stand at the foot of the mountain. Now Mount Sinai was wrapped in smoke, because the Lord had descended upon it in fire; the smoke went up like the smoke of a kiln, while the whole mountain shook violently. As the blast of the trumpet grew louder and louder, Moses would speak and God would answer him in thunder. When the Lord descended upon Mount Sinai, to the top of the mountain, the Lord summoned Moses to the top of the mountain, and Moses went up. Then the Lord said to Moses, "Go down and warn the people not to break through to the Lord to look; otherwise many of them will perish. Even the priests who approach the Lord must consecrate themselves or the Lord will break out against them." Moses said to the Lord, "The people are not permitted to come up to Mount Sinai; for you yourself warned us, saying, 'Set limits around the mountain and keep it holy.' " The Lord said to him, "Go down, and come up bringing Aaron with you; but do not let either the priests or the people break through to come up to the Lord; otherwise he will break out against them." So Moses went down to the people and told them.