Or HaChaim on Genesis 1:6/אור החיים על בראשית א׳:ו׳

1And God said, "let there be a firmament, etc.": The explanation [of firmament] is the firmament that God created on the first day; as I have explained on the verse [starting with] "In the beginning," that everything was created together with one statement, but that the work of each thing was not [yet] refined according to its character. Now God said that it should acquire its place in the waters and that this thing would serve to separate between the waters and the waters. And see [my commentary] to the verse after this one.