Exodus 4:23

and I say to you, Let my son go that he may serve me. If you refuse to let him go , behold, I will kill your firstborn son .

and I told you, “Let my son go, so he may worship me.” But you refused to let him go; so I will kill your firstborn son.`”

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.