How many sons did Abraham have and who were their mothers?
Abraham had two sons. Their mothers were a bondmaid and a free woman. The passage does not name the bondmaid, but it refers back to Genesis where Abram waited 36 years for the promised son and Sarai showed no signs of bearing a child.
Les explains that Paul is referencing the Old Testament account of Abraham, who had two sons—one by a bondmaid and one by a free woman. Les takes us back to Genesis, noting that Abram had waited 36 years for the promise of a son from Genesis 12, and Sarai still had no signs of being able to have a child. He says we shouldn't be too impatient with Abraham, because 36 years is a long time, and he understands why Abraham might have felt God had forgotten the promise.
For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman.