And thou shalt say to Pharaoh, Thus saith Jehovah: Israel is my son, my firstborn. And I say to thee, Let my son go, that he may serve me. And if thou refuse to let him go, behold, I will kill thy son, thy firstborn.
(Exodus 4:22-23 [Darby])
Admittedly, YHWH knew Pharaoh was going to take...