Exodus 19:15-17 New Revised Standard Version (NRSV)
15 And he said to the people, “Prepare for the third day; do not go near a woman.”
16 On the morning of the third day there was thunder and lightning, as well as a thick cloud on the mountain, and a blast of a trumpet so loud that all the people who were in the camp trembled. 17 Moses brought the people out of the camp to meet God. They took their stand at the foot of the mountain.