“And regarding Ishmael, I have heard you; behold, I have blessed him. I will make him fruitful, and I will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation” (Genesis 17:20). “And regarding Ishmael, I have heard you” – Rabbi Yoḥanan in the name of Rabbi Yehoshua ben Ḥanina: The son of the maidservant may be derived from the son of the mistress.16The translation follows the reading of Matnot Kehuna.
“Behold, I have blessed him” – this refers to Isaac. “I will make him fruitful” – this refers to Isaac. “I will multiply him” – this refers to Isaac.17All the blessings of this verse were directed at Isaac, who was mentioned in the previous verse. “And regarding Ishmael” – I have already heard him [and blessed him] through an angel.18The angel who blessed Yishmael when speaking with Hagar (see Genesis 16:10).
I have already blessed Yishmael with all these blessings that I now pronounce upon Isaac. “But My covenant I will keep with Isaac, whom Sarah shall bear you at this designated time next year” (Genesis 17:21). Rabbi Abba bar Kahana in the name of Rabbi Beiri: Here, the son of the mistress may be derived from the son of the maidservant. “Behold, I have blessed him” – this refers to Ishmael.
“I will make him fruitful” – this refers to Ishmael. “I will multiply him” – this refers to Ishmael, and all the more so: “But My covenant I will keep with Isaac.”19The blessings of this verse were directed at Yishmael, but it may be inferred that they are all the more so applicable to Isaac, the son of Hagar’s mistress. Rabbi Yitzḥak said: “All these are the tribes of Israel, twelve” (Genesis 49:28) – these are the descendants of the mistress, but Ishmael would produce only those twelve nesi’im,20As verse 20 states: “Twelve princes [nesi’im] shall he beget.” just as it says: “Clouds [nesi’im], wind, but no rain (Proverbs 25:14).21They are ephemeral and pass by like clouds.
But these are tribes [matot], just as it says: “The oaths to the tribes [matot], an enduring word” (Habakkuk 3:9). “But My covenant I will keep with Isaac, [whom Sarah shall bear you at this designated time [lamo’ed] next year]” – Rabbi Huna in the name of Rabbi Idi – that year was a leap year.22The verse is expounded to mean that Sarah will bear Isaac at the next festival [mo’ed]. The longest interval between festivals is the six months between Sukkot and Passover, but that is not enough time for a pregnancy to come to term.
The Midrash explains that that year was a leap year, so there was a seven-month interval between those holidays, long enough for a viable pregnancy.