After Yitzhak's miraculous birth, Avraham makes a great feast on the day the child is weaned, around age two. Rashi notes it was a great feast because the great ones of the generation attended.

"Sarah saw the son of Hagar the Egyptian, whom she had borne to Avraham, mocking." The word metzachek ("mocking") conceals something grave.

"Cast out this bondwoman and her son, for the son of this bondwoman shall not inherit with my son, with Yitzhak."

To understand, we return to Lekh Lekha. Hagar was Pharaoh's daughter, given to Sarah after the plagues in Egypt. Pharaoh said: "Better to be a servant under Sarah than a princess in my palace."

Pregnant with Ishmael, Hagar begins to despise her mistress: "She conceived, and her mistress was despised in her eyes" (Genesis 16:4). She imagines herself becoming the primary wife.

Sarah reacts at once (Genesis 16:5), asserting a sharp truth: it was she who gave Hagar; a servant she was, a servant she remains. Even the angel, in the desert, addresses her: "Hagar, servant of Sarai, where do you come from?" — and Hagar admits: "I flee from Sarai, my mistress."

The Midrash Pli'a (the "wondrous" midrash) reveals an astonishing chain of cause and effect.

What connects Moses to Ishmael? Sarah was a prophetess. Through Ruach haKodesh she saw that Moses would not enter the Land of Israel — and that very vision prompted her words.

Among the seven Ushpizin of Sukkot, Moses corresponds to the Sefira of Netzach, eternity. Avraham is Chesed, Yitzhak Gevurah, Ya'akov Tiferet, Moses Netzach, Aharon Hod, Yosef Yesod, David Malkhut.

Everything Moses touches becomes eternal: the Torah he transmitted is eternal. Had he entered the Land and built the Temple — "The sanctuary, O Lord, which Your hands established" (Exodus 15:17) — that Temple could never have been destroyed.

Since Moses did not enter, the Ishmaelites would one day contest our Land. The Talmud (Sanhedrin 91a) tells how they summon Israel before Alexander the Great, the greatest power of the age.

Their claim: "Avraham is our father too; Ishmael is his son; so we have a right to the Land's inheritance!" Anxious, the community seeks a defender. A simple Jew, Gevia ben Pesisa, volunteers: "If I win, they'll say a simple Jew defeated them; if I lose, they'll say they sent an insignificant man."

Before Alexander, Gevia opens the Tanakh and reads our very verse: "for the son of this bondwoman shall not inherit." His argument is airtight: Hagar was Sarah's servant, so Ishmael belongs to Sarah; Sarah ruled, Avraham obeyed, Hashem approved — case closed. The Talmud concludes: "the Ishmaelites fled while they still had breath."

Now we grasp Sarah's insistence. Seeing that Moses would not enter and that the Land would one day be contested, she established at that very moment the decisive legal proof: "He does not inherit."

Sarah was protecting not only Yitzhak in the short term, but all of Israel for every generation, until our days and the coming of Mashiach. Our Matriarchs — Sarah, Rivkah, Rachel, and Leah — saw very far and acted for their descendants across all of history.