Exodus 24:15-18

When Moses went up the mountain, the cloud covered it. 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 from the cloud. The appearance of the LORD’s glory to the Israelites was like a consuming fire on the mountaintop. Moses entered the cloud as he went up the mountain, and he remained on the mountain 40 days and 40 nights.