Genesis 21:2

But God said, “No, but Sarah your wife will bear you a son, and you shall call his name Isaac; and I will establish My covenant with him for an everlasting covenant for his seed after him.

“But My covenant I will establish with Isaac, whom Sarah will bear to you at this season next year.”

And He said, “I will surely return to you at this time next year; and behold, Sarah your wife will have a son.” And Sarah was listening at the tent door which was behind him.

“Now Sarah my master’s wife bore a son to my master in her old age, and he has given him all that he has.

“And He gave him the covenant of circumcision; and so Abraham was the father of Isaac, and circumcised him on the eighth day; and Isaac was the father of Jacob, and Jacob of the twelve patriarchs.

For it is written that Abraham had two sons, one by the servant-woman and one by the free woman.

By faith even Sarah herself received ability to conceive, even beyond the proper time of life, since she regarded Him faithful who had promised.