וְיִשְׁמָעֵ֣אל בְּנ֔וֹ בֶּן־שְׁלֹ֥שׁ עֶשְׂרֵ֖ה שָׁנָ֑ה בְּהִ֨מֹּל֔וֹ אֵ֖ת בְּשַׂ֥ר עָרְלָתֽוֹ׃
And Ishmael his son was thirteen years old, when he was circumcised in the flesh of his foreskin.
Read midrashic interpretation of Genesis 17:25: and classical rabbinic exegesis.