Midrash su Genesi 21:27

(Devarim 32:9) "For the portion of the L-rd is His people": An analogy: A king had a field and gave it to tenant-farmers, who began to steal from it — upon which he gave it to their sons — who began to be worse than the first. When a son was born to him, he said to them: Get out of what is mine. You cannot remain there. Give me my portion so that I can recognize it. Similarly, when our father Abraham came to the world, there issued from him base matter — Ishmael and the sons of Keturah. When our father Isaac came to the world, there issued from him base matter — Esav and the chiefs of Edom, who became worse than the first. When Jacob came, no base matter issued from him, but all of his sons were born kasher, as he was, as it is written (Bereshith 21:27) "And Jacob was a whole (i.e., "unalloyed") man, a dweller of tents" (the tents of Torah). From where does the L-rd recognize His portion? From Yaakov, as it is written (Psalms 135:4) "For Yaakov did G-d choose; Israel, as His select ones," and (Devarim, Ibid.) "For the portion of the L-rd is His people, Jacob the cord of His inheritance." And we still do not know whether the L-rd chose Israel as His inheritance, or Israel chose the L-rd, (both readings being possible). It is, therefore, written (Ibid. 7:6) "You (Israel) has the L-rd your G-d chosen to be unto Him a select people."