(Devarim 6:4) "Hear, O Israel, the L-rd our G-d, the L-rd is One": Because it is written (Shemoth 25:2) "Speak to the children of Israel — It is not written "Speak to the children of Abraham and Isaac," but "speak to the children of Israel (i.e., Jacob [viz. Bereshith 32:29]) — Jacob merited that this pronouncement be stated to his children. For Jacob feared all of his days — Woe unto me, lest "base matter" issue from me as it did from my fathers!
From Abraham there came forth Yishmael. From Isaac there came forth Esav my brother. Yishmael was an idolator, viz. (Bereshith 21:9) "And Sarah saw the son of Hagar the Egyptian … 'disporting himself'" (with idolatry). These are the words of R. Akiva.
R. Shimon b. Yochai says: R. Akiva expounds four things where I differ from him, and it seems to me that my view is the correct one: He says "And Sarah the son of Hagar the Egyptian disports himself" with idolatry." And I say that the reference is to the inheritance, Yishmael saying "should I not, being the first-born, take a double portion?" And for this reason Sarah said (Ibid. 10) "Drive out this maidservant and her son." And my view seems more cogent than his.