Exodus 4:21-23

The LORD instructed Moses, “When you go back to Egypt, make sure you do before Pharaoh all the wonders that I have put within your power. But I will harden his heart so that he won’t let the people go. And you will say to Pharaoh: This is what the LORD says: Israel is my firstborn son. I told you: Let my son go so that he may worship me, but you refused to let him go. Look, I am about to kill your firstborn son!”