Rashi on Exodus 19:20/רש"י על שמות י״ט:כ׳

1וירד ה׳ על הר סיני AND THE LORD CAME DOWN UPON MOUNT SINAI — One might think, then, that He actually came down upon it! Therefore it states, (Exodus 20:19) “Ye have seen that I have spoken to you from heaven”. These two texts together teach us that He bent down the upper and lower heavens and spread them out over the top of the mountain like a bed-spread over a bed and the throne of Glory descended upon it (Mekhilta).