וְהִפְרֵתִ֤י אֹֽתְךָ֙ בִּמְאֹ֣ד מְאֹ֔ד וּנְתַתִּ֖יךָ לְגוֹיִ֑ם וּמְלָכִ֖ים מִמְּךָ֥ יֵצֵֽאוּ׃
내가 너로 심히 번성케 하리니 나라들이 네게로 좇아 일어나며 열왕이 네게로 좇아 나리라
Kli Yakar
“Where is Sarah your wife?” Rashi explains that this was to make known that she was modest, in order to endear her to her husband. This explanation is difficult, for did Abraham not know until now that she was modest? It seems to me that since [the angel] said I will surely return to you, implying but not to her, there was concern that perhaps because of this [Abraham] might come to harbor hatred towards her, thinking that she was the reason God had withheld offspring from them. Therefore, the angel saw fit to rectify this and asked Where is Sarah your wife? Although the angel knew where she was, nevertheless he said: “Don’t you acknowledge that she is always in the tent because of her modesty?” If so, surely the Holy One, blessed be He, will not withhold her reward for modesty, for our Sages said (Megillah 10b): “Any bride who is modest merits to have kings descend from her, as proved by Tamar, etc.” She too is modest, therefore her merit led to the promise that kings shall come forth from you (Genesis 17:6) and kings of peoples shall come from her (Genesis 17:16). Through this, her husband would cherish her, as it was her merit of modesty that caused this. Our Sages said (Genesis Rabbah 48:15) that there are dots over the letters alef-yud-vav because they also asked Sarah “Where is he?” And those who interpret hints said [Baal HaTurim] that the letters of “eilav” [to him] and “ayeh” [where] are the same letters as “eleha” [to her] and “ayo” [where is he]. And regarding this, it is certainly difficult [to understand] how they could ask Sarah “where is he” when Abraham was standing right there with them.
Ramban
SHALL A CHILD BE BORN TO HIM THAT IS A HUNDRED YEARS OLD? There is no wonder that a person who is a hundred years old should beget children, for men beget as long as they have vigor even if they be ninety years old or a hundred years old, even in these generations. All the more in the days of Abraham, who had not yet lived two thirds of his life.443Since Abraham lived 175 years (25:7) and was now only 100, he had not yet passed two-thirds of his life-span. So it is no wonder that he was yet in his vigor. Moreover, forty years after this, he begot many children from Keturah.444Further 25:1-2. “Forty years later.” Isaac was wed at the age of forty (25:20), and Abraham married Keturah after Isaac’s marriage (25:1). There was thus an interval of at least forty years between the time Abraham begot Isaac and when he begot children from Keturah. But Abraham’s saying, Shall a child be born to him that is a hundred years old, expressed this thought: Since he did not beget a child from this woman while he was young, how could he beget from her after he was a hundred and she ninety? He knew that her womanly periods had ceased. Therefore Abraham did not say so originally when he was told, And I will make nations of thee,445Above, Verse 6. but only when he was told that he would beget a child from Sarah.
Now He commanded him, And thou shalt call his name Isaac,446Verse 19 here. on account of the rejoicing done by Abraham, which is proof that it was out of faith and joy [that Abraham said, Shall a child be born, etc.]447As explained above. Had it been said out of a sense of derision, G-d would not have told him to call his son by a name which would commemorate his lack of faith. (Bachya.) And after Abraham called him so, as G-d had commanded him, Sarah said, “Is he not rightly named Isaac? For G-d hath made laughter for me.”448Further, 21:6.
Now He commanded him, And thou shalt call his name Isaac,446Verse 19 here. on account of the rejoicing done by Abraham, which is proof that it was out of faith and joy [that Abraham said, Shall a child be born, etc.]447As explained above. Had it been said out of a sense of derision, G-d would not have told him to call his son by a name which would commemorate his lack of faith. (Bachya.) And after Abraham called him so, as G-d had commanded him, Sarah said, “Is he not rightly named Isaac? For G-d hath made laughter for me.”448Further, 21:6.