Exodus 4:21-23

The LORD said to Moses, “When you return to Egypt, see that you perform before Pharaoh all the wonders (miracles) which I have put in your hand, but I will harden his heart and make him stubborn 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 so that he may serve Me’; and if you refuse to let him go, behold, I will kill your son, your firstborn.” ’ ”