Deuteronomy 4:11-13

You approached and stood at the foot of the mountain, and the mountain burned with fire to the [very] heart of the heavens: darkness, cloud and thick gloom. Then the LORD spoke to you from the midst of the fire; you heard the sound of the words, but you saw no form—there was only a voice. So He declared to you His covenant which He commanded you to follow, the Ten Commandments; and He wrote them on two tablets of stone.