Exodus 4:21–23

21 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 heartB,s so that he won’t let the people go. 22 And you will say to Pharaoh: This is what the Lord says: Israel is my firstborn son.t 23 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!”u