Exodus 19:12-16

You shall set barriers for the people all around [the mountain], saying, ‘Beware that you do not go up on the mountain or touch its border; whoever touches the mountain must be put to death. No hand shall touch him [that is, no one shall try to save the guilty party], but the offender must be stoned or shot through [with arrows]; whether man or animal [that touches the mountain], he shall not live.’ When the ram’s horn sounds a long blast, they shall come up to the mountain.” [Num 24:8] So Moses went down from the mountain to the people and sanctified them [for God’s sacred purpose], and they washed their clothes. He said to the people, “Be prepared for the third day; do not be intimate with a woman.” So it happened on the third day, when it was morning, that there were thunder and flashes of lightning, and a thick cloud was on the mountain, and a very loud blast was sounded on a ram’s horn, so that all the people who were in the camp trembled.