Exodus 19:12-16

You must set boundaries for the people all around, saying, ‘Take heed to yourselves not to go up on the mountain nor touch its edge. Whoever touches the mountain will surely be put to death! No hand will touch him – but he will surely be stoned or shot through, whether a beast or a human being; he must not live.’ When the ram’s horn sounds a long blast they may go up on the mountain.” Then Moses went down from the mountain to the people and sanctified the people, and they washed their clothes. He said to the people, “Be ready for the third day. Do not go near your wives.” On the third day in the morning there was thunder and lightning and a dense cloud on the mountain, and the sound of a very loud horn; all the people who were in the camp trembled.