Exodus 4:21-23

Darby Translation

²¹ And Jehovah said to Moses, When thou goest to return to Egypt, see that thou do all the wonders before Pharaoh that I have put in thy hand. And I will harden his heart, that he shall not let the people go.

²² And thou shalt say to Pharaoh, Thus saith Jehovah: Israel is my son, my firstborn.

²³ 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.