Exodus 19:12; Exodus 19:16

12 "You shall set bounds 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 surely be put to death.

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