Deuteronomy 4:11-15

You came forward and stood at the foot of the mountain, and the mountain was burning with fire to the 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 words, but you saw no form—there was only a voice.

So He declared to you His covenant which He commanded you to perform, that is, the Ten Commandments; and He wrote them on two tablets of stone.

The Lord commanded me at that time to teach you statutes and judgments, so that you would perform them in the land where you are going over to take possession of it.

“So be very careful yourselves, since you did not see any form on the day the Lord spoke to you at Horeb from the midst of the fire,