Exodus 4:21-23

Amplified Bible

²¹ The Lord said to Moses, “When you return to Egypt, see that you perform before Pharaoh all the wonders (miracles) which I have put in your hand, but I will harden his heart and make him stubborn so that he will not let the people go. ²² Then you shall say to Pharaoh, ‘Thus says the Lord, “Israel is My son, My firstborn. ²³ So I say to you, ‘Let My son go so that he may serve Me’; and if you refuse to let him go, behold, I will kill your son, your firstborn.”’”