Once there was a king who decreed: The people of Rome are forbidden to go down to Syria, and the people of Syria are forbidden to go up to Rome. Likewise, when G-d created the world He decreed and said: "The heavens are G-d's, and the earth is given to man." But when He wished to give the Torah to Israel, He rescinded His original decree, and declared: The lower realms may ascend to the higher realms, and the higher realms may descend to the lower realms. And I, Myself, will begin -- as it is written, "And G-d descended on Mount Sinai," and then it says, "And to Moses He said: Go up to G-d."
–Midrash Tanchuma, Vaeira 15