Musar zu Bereschit 16:11

G–d called upon Rahav, the guardian angel of Ishmael, asking whether he wanted to accept the Torah. Rahav also wanted to know what is written in the Torah; G–d told him that the Torah prohibits adultery. This proved unacceptable to the representative of Ishmael who considered his whole kingdom on earth as based on the blessing of water, i.e. the blessing given to the fish in Genesis 1,22 to multiply indiscriminately without regard to exclusive sexual pairings. When the angel told Hagar that she would give birth to Ishmael (Genesis 16,11), he told her that Ishmael would be פרא אדם, "an unbridled human being." [the word פרא here seems related to פרו in the way the Zohar uses it. Ed.] Rahav began to plead with G–d: "Abraham had two sons.; You have the choice of giving the Torah to one of Isaac's two sons. The Torah is far more suitable for either of them than for Ishmael." G–d responded that this would not be fair because Ishmael was Abraham's first-born." The guardian angel of Ishmael then offered to waive his claim to the birthright of Ishmael in favour of Isaac. He offered to also cede to Isaac and his descendants the "light" Ishmael had inherited by dint of being Abraham's first-born son. G–d accepted this offer, and this is why the Torah says: הופיע מהר פארן.