Flavius Josephus, Antiquities of the Jews
William Whiston, A.M., Ed.

[79] So they passed two days in this way of feasting; but on the third day, before the sun was up, a cloud spread itself over the whole camp of the Hebrews, such a one as none had before seen, and encompassed the place where they had pitched their tents; and while all the rest of the air was clear, there came strong winds, that raised up large showers of rain, which became a mighty tempest. There was also such lightning, as was terrible to those that saw it; and thunder, with its thunderbolts, were sent down, and declared God to be there present in a gracious way to such as Moses desired he should be gracious. Now, as to these matters, every one of my readers may think as he pleases; but I am under a necessity of relating this history as it is described in the sacred books. This sight, and the amazing sound that came to their ears, disturbed the Hebrews to a prodigious degree, for they were not such as they were accustomed to; and then the rumor that was spread abroad, how God frequented that mountain, greatly astonished their minds, so they sorrowfully contained themselves within their tents, as both supposing Moses to be destroyed by the Divine wrath, and expecting the like destruction for themselves.