Bereshit Rabbah 4:2

"And God said: Let there be a firmament in the midst of the waters" (Genesis 1:6). The Rabbis said it in the name of Rabbi Chanina, and Rabbi Pinchas and Rabbi Yaakov son of Rabbi Avin in the name of Rabbi Shmuel bar Nachman: At the moment the Holy One, blessed be He, said "Let there be a firmament in the midst of the waters," the middle drop congealed, and the lower heavens and the heaven of heavens above were made. Rav said: On the first day their substance was moist, and on the second day it congealed. "Let there be a firmament" let the firmament grow firm. Rabbi Yehuda son of Rabbi Simon said: Let a patch be made for the firmament, just as you say, "And they beat the gold into thin plates" (Exodus 39:3). Rabbi Chanina said: Fire went forth from above and licked the surface of the firmament. Rabbi Yochanan, when he would reach this verse, "By His breath the heavens were made fair" (Job 26:13), would say: Rabbi Chanina taught me this well. Rabbi Yudan son of Rabbi Shimon said: Fire went forth from above and scorched the surface of the firmament. Rabbi Berekhya in the name of Rabbi Abba bar Kahana said: The work of creation came to teach about the giving of the Torah, and ended up learning from it, "As when the melting fire burns" (Isaiah 64:1). When did the fire divide between the upper and the lower waters? Was it not at the giving of the Torah? I wonder. Yet so it was at the creation of the world.