Exodus 19:12-16

But you shall set boundaries for the people all around, saying, ‘Beware that you do not go up on the mountain or touch the border of it; whoever touches the mountain shall certainly be put to death.

No hand shall touch him, but he shall certainly be stoned or shot through; whether animal or person, the violator shall not live.’ When the ram’s horn 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.

He also said to the people, “Be ready for the third day; do not go near a woman.”

So it came about on the third day, when it was morning, that there were thunder and lightning flashes and a thick cloud over the mountain and a very loud trumpet sound, so that all the people who were in the camp trembled.