Genesis 17:19

“So the bow shall be in the cloud, and I will look upon it, to remember the everlasting covenant between God and every living creature of all flesh that is on the earth.”

“And I will establish My covenant between Me and you and your seed after you throughout their generations for an everlasting covenant, to be God to you and to your seed after you.

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

And Abraham called the name of his son who was born to him, whom Sarah bore to him, Isaac.

As He spoke to our fathers, To Abraham and his seed forever.”

“For this reason Moses has given you circumcision (not because it is from Moses, but from the fathers), and on the Sabbath you circumcise a man.

“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.

But the son by the servant-woman had been born according to the flesh, while the son by the free woman through the promise.

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