The marvel of the birth of Isaac to his parents when they were of such an advanced age, Abraham when he was one hundred years old and Sarah ninety, led to skepticism and gossip among the people, until it was clearly proven that Isaac was the son of Abraham and Sarah.

“[Sarah] said: Who would have said of Abraham that Sarah would nurse children?” (Genesis 21:7). Why is “children” written in plural; how many children did she nurse? Rabbi Levi said: On the day that Abraham celebrated the weaning of his son Isaac, he made a great feast. All the nations of the world were gossiping and saying: Did you see the old man and old woman who brought a foundling from the street and claim: He is our son? Moreover, they make a great feast to bolster their claim. What did Abraham our patriarch do? He went and invited all the prominent men of the generation, and Sarah invited their wives. Each and every one brought her baby with her, but did not bring her wet nurse. A miracle befell Sarah our matriarch and her breasts opened like two springs. Milk flowed from them and she nursed all of them. But they were still gossiping and saying: Even if Sarah, at the age of ninety, could give birth, could Abraham, at the age of one hundred, father a child? Immediately, the countenance of Isaac’s face was transformed and he resembled Abraham. All of them began to say: “Abraham begot Isaac” (25:19). (Bava Metzia 87a)