Exodus 4:21-23

And the LORD said to Moses, “When you go back to Egypt, see that you do all those wonders before Pharaoh which I have put in your hand. 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 son, My firstborn. So I say to you, let My son go that he may serve Me. But if you refuse to let him go, indeed I will kill your son, your firstborn.” ’ ”