Exodus 24:16-17 (ESV)

And the glory of the Lord settled on Mount Sinai... For six days the cloud covered it, and on the seventh day the Lord called to Moses from within the cloud. And the sight of the glory of the Lord was like a consuming fire on the mountaintop in the eyes of the Israelites.

Exodus 24 16 17 Dore

Moses on Mount Sinai, 1866
Gustav Doré (French, 1832-1883)