Exodus 4:22-24 King James Version (KJV)

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.

Exodus 4