Exodus 4:21-23

4:21 The Lord said to Moses, “When you go back to Egypt, see that you do before Pharaoh all the wonders I have put under your control. But I will harden his heart and he will not let the people go. 4:22 You must say to Pharaoh, ‘Thus says the Lord, “Israel is my son, my firstborn, 4:23 and I said to you, ‘Let my son go that he may serve me,’ but since you have refused to let him go, I will surely kill your son, your firstborn!”’”