17:25י״ז:כ״ה
1 א

בהמולו את בשר ערלתו. בְּאַבְרָהָם לֹא נֶאֱמַר "אֵת", לְפִי שֶׁלֹּא הָיָה חָסֵר אֶלָּא חִתּוּךְ בָּשָׂר, שֶׁכְּבָר נִתְמָעֵךְ עַל יְדֵי תַשְׁמִישׁ; אֲבָל יִשְׁמָעֵאל שֶׁהָיָה יֶלֶד, הֻזְקַק לַחְתֹּךְ עָרְלָה וְלִפְרֹעַ הַמִּילָה, לְכָךְ נֶאֱמַר בּוֹ "אֵת" (בראשית רבה):

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)