Exodus 4:21-23 NIV

21 The LORD said to Moses, "When you return to Egypt, see that you perform before Pharaoh all the wonders1 I have given you the power to do. But I will harden his heart2 so that he will not let the people go.3
22 Then say to Pharaoh, 'This is what the LORD says: Israel is my firstborn son,4

References for Exodus 4:22

23 and I told you, "Let my son go,5 so he may worship6 me." But you refused to let him go; so I will kill your firstborn son.' "7

References for Exodus 4:23