Midrash sa Isaias 64:2: Mekhilta DeRabbi Yishmael

(Exodus 19:20) "And the L–rd descended upon Mount Sinai": I might think that the "Glory" itself descended on Mount Sinai. It is, therefore, written (Ibid. 20:19) "that from the heavens I spoke to you." We are hereby apprised that the Holy One Blessed be He bent the lower heavens and the upper heavens of heaven and spread them over the top of the mountain and the Glory descended on top of the mountain. (He did this) as one spreads a bolster on a bed, (and He spoke) as a man speaking from a bolster, viz. (Isaiah 63:19-64:1) "Had You not split the heavens and descended, the mountains quaking before You, as fire kindles brushwood, fire boiling water, etc."