His son by the slave woman was born according to the flesh, but his son by the free woman was born through the promise.

Genesis 17:16
And I will bless her and will surely give you a son by her. I will bless her, and she will be the mother of nations; kings of peoples will descend from her."

Genesis 18:10
Then the LORD said, "I will surely return to you at this time next year, and your wife Sarah will have a son!" Now Sarah was behind him, listening at the entrance to the tent.

Genesis 21:1
Now the LORD attended to Sarah as He had said, and the LORD did for Sarah what He had promised.

Romans 9:7
Nor because they are Abraham's descendants are they all his children. On the contrary, "Through Isaac your offspring will be reckoned."

Galatians 4:22
For it is written that Abraham had two sons, one by the slave woman and the other by the free woman.

Galatians 4:28
Now you, brothers, like Isaac, are children of promise.

Galatians 4:29
At that time, however, the son born by the flesh persecuted the son born by the Spirit. It is the same now.

Hebrews 11:11
By faith Sarah, even though she was barren and beyond the proper age, was enabled to conceive a child, because she considered Him faithful who had promised.

But he who was of the female slave was born after the flesh; but he of the free woman was by promise.