Yalkut Shimoni on Torah 82:6

"And God said to Abraham, As for Sarai your wife, you shall not call her name Sarai" (Genesis 17:15). "A woman of valor is the crown of her husband" (Proverbs 12:4). Rabbi Aha said, her husband was crowned through her, but she was not crowned through her husband. The Rabbis say, she was the mistress of her household. Everywhere the man decrees, but here, "In all that Sarah says to you, listen to her voice" (Genesis 21:12). Rabbi Yehoshua ben Korchah said, the yod that the Holy One, blessed be He, took from Sarai was divided, half to Sarah and half to Abraham. Rabbi Shimon ben Yochai said, the yod that the Holy One, blessed be He, took from Sarai went flying and hovering around the throne of the Holy One, blessed be He, and said before Him: Master of the universe, because I am the smallest of the letters You have removed me from the name of a righteous woman! The Holy One, blessed be He, said to her: In the past you were set in a feminine name and at the end of the letters; now you will be set in a masculine name and at the head of the letters. This is what is written, "And Moses called Hoshea son of Nun, Yehoshua" (Numbers 13:16). Rabbi Mani said, in the past she was Sarai [princess] to her own people, and now she is Sarah [princess] to all who enter the world. "And I will bless her" (Genesis 17:16). Rabbi Yehudah says, "and I will bless her" means to give her a son; "and I will bless her" means to bless her with milk. Rabbi Nechemiah said to him, but had she not already received the good tidings? Rather, it teaches that the Holy One, blessed be He, restored her to the days of her youth. Rabbi Abahu in the name of Rabbi Yose bar Chanin: I place reverence for her upon all the nations of the world, that they should not taunt her and call her barren. Rabbi Yudan in the name of Rabbi Shimon ben Lakish: she had no womb at all, and the Holy One, blessed be He, fashioned a womb for her. "Kings of peoples shall be from her" - from here Abraham expounded and brought back Keturah. "And Abraham fell on his face and laughed" (Genesis 17:17). Rabbi Pinchas in the name of Rabbi Levi: twice Abraham fell on his face, and corresponding to them circumcision was taken from his children twice, once in Egypt and once in the wilderness. In Egypt Moses came and circumcised them; in the wilderness Joshua came and circumcised them. "Shall a child be born to one a hundred years old" - and why? Because "shall Sarah, who is ninety years old, bear?" The man does not age but the woman ages. Who is called aged? Anyone whom they call "so-and-so's mother" and she does not object. "O that Ishmael might live before You" (Genesis 17:18). This is like a king's friend who used to pay him a tribute, and the king said to him, I wish to double your tribute. He said to him, do not let my spirit grow cold; would only that you do not withhold from me the first amount. So too, "O that Ishmael might live before You." "And as for Ishmael I have heard you, behold I have blessed him" (Genesis 17:20). Rabbi Yochanan said, here the son of the maidservant learns from the son of the mistress: "behold I have blessed him" - this is Isaac; "and I will multiply him" - this is Isaac; "and I will make him fruitful" - this is Isaac. As for Ishmael, I have already heard him by way of an angel. Rav Abba bar Kahana in the name of Rav Biri: here the son of the mistress learns from the son of the maidservant: "behold I have blessed him" - this is Ishmael; how much more so, "and I will establish My covenant with Isaac." "Twelve princes shall he beget" - it is written, "All these are the tribes of Israel, twelve" (Genesis 49:28); and Ishmael does not raise up twelve such, but those are merely "princes" [nesi'im], as you say, "clouds [nesi'im] and wind and no rain" (Proverbs 25:14); but these are staffs, as you say, "sworn are the staffs of the word, selah" (Habakkuk 3:9).