Exodus 4:21-23

Revised Standard Version

²¹ And the Lord said to Moses, “When you go back to Egypt, see that you do before Pharaoh all the miracles which I have put in your power; but I will harden his heart, so that he will not let the people go. ²² And you shall say to Pharaoh, ‘Thus says the Lord, Israel is my first-born son, ²³ and I say to you, “Let my son go that he may serve me”; if you refuse to let him go, behold, I will slay your first-born son.’”