Deuteronomy 4:11-13

And ye came near and stood under the mountain; and the mountain burned with fire unto the heart of heaven, with darkness, cloud, and thick darkness. And Jehovah spake unto you out of the midst of the fire: ye heard the voice of words, but ye saw no form; only ye heard a voice. And he declared unto you his covenant, which he commanded you to perform, even the ten commandments; and he wrote them upon two tables of stone.