Exodus 19:10-19 (Holy Thursday Vespers)
And the LORD said to Moses, "Go to the people and consecrate them today and tomorrow, and let them wash their garments, and be ready by the third day; for on the third day the LORD will come down upon Mount Sinai in the sight of all the people. And you shall set bounds for the people round about, saying, 'Take heed that you do not go up into the mountain or touch the border of it; whoever touches the mountain shall be put to death; no hand shall touch him, but he shall be stoned or shot; whether beast or man, he shall not live.' When the trumpet sounds a long blast, they shall come up to the mountain." So Moses went down from the mountain to the people, and consecrated the people; and they washed their garments. And he said to the people, "Be ready by the third day; do not go near a woman." On the morning of the third day there were thunders and lightnings, and a thick cloud upon the mountain, and a very loud trumpet blast, so that all the people who were in the camp trembled. Then Moses brought the people out of the camp to meet God; and they took their stand at the foot of the mountain. And Mount Sinai was wrapped in smoke, because the LORD descended upon it in fire; and the smoke of it went up like the smoke of a kiln, and the whole mountain quaked greatly. And as the sound of the trumpet grew louder and louder, Moses spoke, and God answered him in thunder.