Exodus 4:21-23

And the LORD told Moses, “When you arrive back in Egypt, go to Pharaoh and perform all the miracles I have empowered you to do. But I will harden his heart so he will refuse to let the people go. Then you will tell him, ‘This is what the LORD says: Israel is my firstborn son. I commanded you, “Let my son go, so he can worship me.” But since you have refused, I will now kill your firstborn son!’”