Genesis 17:27

So Abram heard that his relative had been taken captive, and he led out his trained men, born in his house, 318 in number, and went in pursuit as far as Dan.

“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.