Tafsiran Kejadian 17:25: Rashi, Chizkuni, Radak & Siftei Chakhamim

בהמולו את בשר ערלתו WHEN HE HAD THE FLESH OF HIS FORESKIN CIRCUMCISED — With Avraham, [the word], et is not stated [as it is here with Yishmael], because the only thing he was missing was the cutting of the flesh, as [his foreskin] was already flattened through sexual relations. But Yishmael - who was only a child - was required to cut his foreskin and open [and push back] the circumcision. Hence about him, et is stated. (Genesis Rabbah 47:8)

וישמעאל בנו בן שלש עשרה שנה וגו , “and Ishmael his son who was 13 years old at the time, etc;” the details provided in this verse are meant to teach us that Avraham as well as his son were circumcised on the same day. We derive this from the expression: בעצם היום הזה, as elsewhere the Torah had told us that Avraham had been ninety nine and the Torah records that Avraham was eighty six years old on the day when Hagar gave birth to Ishmael, and that he was ninety nine years old when he circumcised himself. The only possible reason why the Torah had to be so precise about both data was to teach us that just as Avraham had not delayed with carrying out G-d’s commandment, so also Ishmael submitted to the same commandment without delay.

וישמעאל בנו בן שלש עשרה שנה, seeing the Torah had mentioned Avraham's age at the time of his circumcision, Ishmael's age was also mentioned as a mark of honour for him. After all, we know how old Ishmael was at the time, the Torah having told us that Avraham his father was 86 years old when he was born (Genesis 15,16). Perhaps the Torah underlines the fact that Avraham circumcised his son Ishmael, as if he had not been circumcised by that age, he himself is obligated to either circumcise himself or to arrange for him circumcision. Ishmael was just young enough for this obligation still to be that of this father.