Exodus 4:21-23

The LORD said to Moses, “When you get back to Egypt, do all the miracles I have given you the power to do. Show them to the king of Egypt. But I will make the king very stubborn, and he will not let the people go. Then say to the king, ‘This is what the LORD says: Israel is my firstborn son. I told you to let my son go so he may worship me. But you refused to let Israel go, so I will kill your firstborn son.’ ”