Deuteronomy 4:15–16 (ESV)

15 “Therefore watch yourselves very carefully. Since you saw no form on the day that the LORD spoke to you at Horeb out of the midst of the fire,

In this detailed warning against the dangers of idolatry (Deuteronomy 4:15–19), Moses begins with the principle. When Israel had its awesome encounter with the Lord on Sinai, it did not see any form, though the reality of God's presence was certain. Nevertheless, the temptation to resort to physical representations of God was real, and already experience at the incident of the golden calf. Therefore, Moses warns Israel to keep careful watch, lest they act corruptly by making a carved image for themselves.