Exodus 24:15-18

And when Moses had ascended, a cloud covered the mountain. And the glory of the Lord dwelt upon Sinai, covering it with a cloud for six days. And on the seventh day, he called to him from the middle of the mist. Now the appearance of the glory of the Lord was like a burning fire over the summit of the mountain in the sight of the sons of Israel. And Moses, entering into the midst of the cloud, ascended the mountain. And he was there for forty days and forty nights.