Exodus 4:23 in the World English Bible
and I have said to you, “Let my son go, that he may serve me;” and you have refused to let him go. Behold, I will kill your firstborn son.’”
Context
22 And thou shalt say unto Pharaoh, Thus saith the LORD, Israel is my son, even my firstborn:
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.
24 And it came to pass by the way in the inn, that the LORD met him, and sought to kill him.