Exodus 4:21-23

And the LORD said to Moses, “When you go back to Egypt, see that you do before Pharaoh all the miracles that I have put in your power. But I will harden his heart, so that he will not let the people go. Then you shall say to Pharaoh, ‘Thus says the LORD, Israel is my firstborn son, and I say to you, “Let my son go that he may serve me.” If you refuse to let him go, behold, I will kill your firstborn son.’”