Exodus 4:23

And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn.

If thou refuse ... I will slay thy son, even thy first-born. The death of the first-born in the palace, and throughout the land of Egypt, as an act of retribution for refusing His first-born to God, would bring home in an emphatic manner to the business and bosoms of the Egyptians the full import of the phrase, "Israel is my first-born."