[21] And the LORD said unto Moses, to return into Egypt, see that thou do all those wonders before Pharaoh, which I have put in thine hand: but I will harden his heart, that he shall not let the people go. [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.