Charles Ellicott Commentary Exodus 4:23

"and I have said unto thee, Let my son go, that he may serve me; and thou hast refused to let him go: behold, I will slay thy son, thy first-born." — Exodus 4:23 (ASV)

I will slay thy son, even thy first-born. —The threat was not made until immediately before the tenth plague (Exodus 11:5). It is not recorded in the words which Moses is here directed to use; but the speech of Moses in Exodus 11 is no doubt much abbreviated.