Albert Barnes Commentary Exodus 12:43

"And Jehovah said unto Moses and Aaron, This is the ordinance of the passover: there shall no foreigner eat thereof;" — Exodus 12:43 (ASV)

And the Lord said - From this verse to Exodus 13:16 are instructions regarding the Passover. Such instructions were needed when the Israelites were joined by the “mixed multitude” of strangers; and they were probably given at Succoth, on the morning following the departure from Rameses.

No stranger - Literally, “son of a stranger.” The term is general; it includes all who were aliens from Israel, until they were incorporated into the nation by circumcision.