Rashi on Genesis 1:6/רש"י על בראשית א׳:ו׳

1יהי רקיע BE THERE AN EXPANSE — Let the expansion become fixed; for although the heavens were created on the first day, they were still in a fluid form, and they became solidified only on the second day at the dread command (literally, rebuke) of the Holy One, blessed be He, when he said “Let the firmament be stable” (Genesis Rabbah 4:2). It is to this that allusion is made in what is written in (Job 26:11): “The pillars of heaven were trembling” (i. e. they were unstable) — this was during the whole of the first day — and on the second (Job 26:11): “they were astonished at His rebuke”, like a man who stands immovable, amazed at the rebuke of one who terrifies him.

2בתוך המים IN THE MIDST OF THE WATERS — In the exact centre of the waters; because there is the same distance between the upper waters and the firmament as there is between the firmament and the waters that are upon the earth. Thus you may infer that they (the upper waters) are suspended in space by the command of the King (Genesis Rabbah 4:3).