Exodus 4:21-23

The LORD said to Moses, “When you go back to Egypt see that you perform before Pharaoh all the wonders which 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 son, My firstborn. So I said to you, ‘Let My son go that he may serve Me’; but you have refused to let him go. Behold, I will kill your son, your firstborn.” ’ ”