מדרש אויף בראשית 17:21

These are the generations of Isaac, Abraham’s son: Abraham begot Isaac (Gen. 25:19). Scripture states elsewhere in allusion to this verse: The father of the righteous will greatly rejoice; and he that begetteth a wise child will have joy of him (Prov. 23:24). Concerning whom did Solomon make this statement? He said it about Isaac. When Isaac was born, the entire universe rejoiced; the heavens and the earth, the sun and the moon, the stars and the planets. Why did they rejoice? Because the world would not have survived had Isaac not been born, as it is said: If My covenant be not with day and night, if I have not appointed the ordinance of heaven and earth (Jer. 33:25). My covenant refers to Isaac, as it is said: I will establish My covenant with Isaac (Gen. 17:21).

… And He said to him ‘go away to the land of Moriah and bring him up there for a burnt offering’ (Bereshit 22:2) What is the land of Moriah? There is a whole bundle of Sages here, each saying their own answer. R’ Yanai says ‘what is Moriah? The place from which awe and fear (morah and yirah) go out to the world,’ “You are feared, O God, from Your Sanctuary…” (Tehillim 68:36) R’ Chiya the elder says ‘the land from which instruction (hora’ah) goes out to the world,’ as it says “…for out of Zion shall the Torah come forth…” (Yeshayahu 2:3) Another explanation: the land from which, in the future, the Holy One will teach that the wicked should descend to gehinnom, as it says “Like sheep, they are destined to the grave; death will devour them, and the upright will rule over them in the morning, and their form will outlast the grave…” From where? “…his dwelling place (zevul).” (Tehillim 49:15) Another explanation of the land of Moriah. R’ Yehoshua ben Levi said ‘the land from which the righteous teach (morim) and make decrees upon the Holy One which He does,’ as it says “…and David and the elders, covered with sackcloth, fell upon their faces. And David said to God, "Did I not say to count the people?…I beg that Your hand be against me and against my father's house, but not against Your people for a plague." (Divre HaYamim I 21:16-17) Another explanation of the land of Moriah. R’ Yehudah bar Padiiya said ‘Moriah - he said to Gd, where is it? He replied – to the land which I will show (mareh) you.’ Another explanation of Moriah. Avraham said to Gd, Master of the World! But am I fit to offer sacrifices? Am I a kohen? Let Shem the High Priest come and receive him from me. The Holy One replied to him – when you arrive at the place I will sanctify you and make you into a kohen. What is the meaning of Moriah? In exchange (temurah) for Shem. His replacement, as it says “He shall not exchange it or offer a substitute for it…” (Vayikra 27:10) Another explanation. What is Moriah? R’ Pinchas said ‘the land in which the master (maruto) of the world dwells,’ as it says “…and My eyes and My heart shall be there at all times.” (Melachim I 9:3) Another explanation. What is Moriah? R’ Shimon bar Yochai said ‘the land which was adorned opposite the altar above “…or cast down… (yaro yireh)” (Shemot 19:13) Another explanation. The land in which the incense is offered – “I will go to the mountain of myrrh (mor)…” (Shir HaShirim 4:6)

“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.