Exodus 12:48

ועשה פסח AND HE WILL KEEP THE PASSOVER (the words may mean, then he shall keep the Passover) — One might think that this verse implies that any-one who becomes a proselyte (גר) must keep the Passover-offering rite immediately after his circumcision (even though this has not taken place just before Passover), therefore Scripture states והיה כאזרח הארץ “but he shall be as a native of the land”. How is it in the case of a native? He brings the offering on the fourteenth! So, too, a proselyte must bring it only on the fourteenth (Mekhilta). וכל ערל לא יאכל בו AND NO UNCIRCUMCISED PERSON SHALL EAT THEREOF — This is stated in addition to the somewhat similar text in v. 43 in order to include in the prohibition of eating the Paschal offering any person whose brothers have died in consequence of circumcision (in which case the parents are exempted from circumcising any of their children born after the death of these), because such a one is not to be regarded as an apostate who of set purpose remains uncircumcised, and the law regarding such a one cannot be derived from the statement in v. 43, “no strange person shall eat thereof” (because Rashi has explained this to refer to an Israelite whose doings have estranged him from God).

וְכִֽי־יָג֨וּר אִתְּךָ֜ גֵּ֗ר וְעָ֣שָׂה פֶ֘סַח֮ לַיהֹוָה֒ הִמּ֧וֹל ל֣וֹ כׇל־זָכָ֗ר וְאָז֙ יִקְרַ֣ב לַעֲשֹׂת֔וֹ וְהָיָ֖ה כְּאֶזְרַ֣ח הָאָ֑רֶץ וְכׇל־עָרֵ֖ל לֹֽא־יֹ֥אכַל בּֽוֹ׃

And when a stranger shall sojourn with thee, and will keep the passover to the Lord, let all his males be circumcised, and then let him come near and keep it; and he shall be as one that is born in the land: for no uncircumcised person shall eat of it.