Exodus 4:21-23

New Revised Standard Version, Anglicised

²¹ And the Lord said to Moses, ‘When you go back to Egypt, see that you perform before Pharaoh all the wonders that I have put in your power; but I will harden his heart, so that he will not let the people go. ²² Then you shall say to Pharaoh, “Thus says the Lord: Israel is my firstborn son. ²³ I said to you, ‘Let my son go that he may worship me.’ But you refused to let him go; now I will kill your firstborn son.”’