and his son Ishmael was thirteen;
We have here Abraham's obedience to the law of circumcision. He himself and all his family were circumcised, so receiving the token of the covenant and distinguishing themselves from other families, that had no part nor lot in the matter. 1. It was an implicit obedience: He did as God had said to him, and did not ask why or wherefore.
Commenting on Genesis 17:23-27
And Ishmael his son was thirteen years old when he was circumcised in the flesh of his foreskin. Hence the Arabians, as Josephus (w) relates, circumcise their children when at thirteen years of age, because Ishmael, the founder of their nation, was circumcised at that age; and Origen (x) asserts the same; and with which agrees what an Arabic writer says (y) of the Arabians...
Gen 17:22-27 When God had finished His address and ascended again, Abraham immediately fulfilled the covenant duty enjoined upon him, by circumcision himself on that very day, along with all the male members of his house. Because Ishmael was 13 years old when he was circumcised, the Arabs even now defer circumcision to a much later period than the Jews, generally till between the ages...
Commenting on Genesis 17:22-27