Exodus 24:15-18 NRSV
Then Moses went up on the mountain, and the cloud covered the mountain. The glory of the LORD settled on Mount Sinai, and the cloud covered it for six days; on the seventh day he called to Moses out of the cloud. Now the appearance of the glory of the LORD was like a devouring fire on the top of the mountain in the sight of the people of Israel. Moses entered the cloud, and went up on the mountain. Moses was on the mountain for forty days and forty nights.
NRSV: New Revised Standard Version