Exodus 24:15-18 NIV
When Moses went up on the mountain, the cloud covered it, and the glory of the LORD settled on Mount Sinai. For six days the cloud covered the mountain, and on the seventh day the LORD called to Moses from within the cloud. To the Israelites the glory of the LORD looked like a consuming fire on top of the mountain. Then Moses entered the cloud as he went on up the mountain. And he stayed on the mountain forty days and forty nights.
NIV: New International Version