Deuteronomy 4:12

And the Lord spake unto you out of the midst of the fire
For the Lord descended on Mount Sinai in a cloud, in fire, and was in the thick darkness, from whence he delivered out the ten commands:

ye heard the voice of the words;
distinctly and plainly, not only the sound of them, but the words themselves, and so as to understand what was meant by them;

but saw no similitude;
not any likeness of the person speaking, by which they could form any idea of him in their minds, which was purposely done to prevent idolatry:

only ye heard a voice;
that was all.