"It was taught: The light which was created in the six days of Creation (the light which came into existence at God's command, not that shed by the sun) cannot illumine by day, because it would eclipse the light of the sun, nor by night, because it was created only to illumine by day. Then where is it? It is stored up for the righteous in the Messianic future, as it says, Moreover the light of the moon shall be as the light of the sun, and the light of the sun shall be sevenfold, as the light of the seven days (Isa. XXX, 26)."
Midrash Rabbah, Genesis (Bereshith) III. 6