And the LORD spoke to you out of the midst of the fire. You heard the sound of the words, but saw no form; you only heard a voice.
Deuteronomy 4:12
Surrounding context
4:11 "Then you came near and stood at the foot of the mountain, and the mountain burned with fire to the midst of heaven, with darkness, cloud, and thick darkness.
4:12 And the LORD spoke to you out of the midst of the fire. You heard the sound of the words, but saw no form; you only heard a voice.
4:13 So He declared to you His covenant which He commanded you to perform, the Ten Commandments; and He wrote them on two tablets of stone.