Exodus 4:21-23

And Jehovah said unto Moses, When thou goest back into Egypt, see that thou do before Pharaoh all the wonders which I have put in thy hand: but I will harden his heart and he will not let the people go. And thou shalt say unto Pharaoh, Thus saith Jehovah, Israel is my son, my first-born: and I have said unto thee, Let my son go, that he may serve me; and thou hast refused to let him go: behold, I will slay thy son, thy first-born.