Exodus 4:21-23

Amplified Bible, Classic Edition

²¹ And the Lord said to Moses, When you return into Egypt, see that you do before Pharaoh all those miracles and wonders which I have put in your hand; but I will make him stubborn and harden his heart, so that he will not let the people go.

²² And you shall say to Pharaoh, Thus says the Lord, Israel is My son, even My firstborn.

²³ And I say to you, Let My son go, that he may serve Me; and if you refuse to let him go, behold, I will slay your son, your firstborn.