Rashi on Genesis 17:16/רש"י על בראשית י״ז:ט״ז

1וברבתי אותה AND I WILL BLESS HER — And in what did the blessing consist? In that she resumed her youthfulness, as it is said, (Genesis 18:12) “shall I have the pleasure of youth again?” (Genesis Rabbah 47:2)

2וברכתיה AND I WILL BLESS HER with teeming breasts. And this happened when she really needed this on the day of Isaac’s feast. For people were speaking spitefully about them, that they had adopted a foundling from the street saying, “This is our son.” Every one, therefore, brought her child with her, but did not bring the wet-nurse and she (Sarah) suckled all of them. To this refers that which is said (Genesis 21:7) “Sarah has given suck to children”. In Genesis Rabbah 47:2 this is alluded to in part.