Exodus 4:21-23

The LORD said to Moses, “When you return to Egypt, see that you perform before Pharaoh all the wonders I have given you the power to do. But I will harden his heart so that he will not let the people go. Then say to Pharaoh, ‘This is what the LORD says: Israel is my firstborn son, and I told you, “Let my son go, so he may worship me.” But you refused to let him go; so I will kill your firstborn son.’ ”