Exodus 4:22 in the King James Version
And thou shalt say unto Pharaoh, Thus saith the LORD, Israel is my son, even my firstborn:
Context
21 The LORD said to Moses, “When you go back into Egypt, see that you do before Pharaoh all the wonders which I have put in your hand, but I will harden his heart and he will not let the people go.
22 You shall tell Pharaoh, ‘The LORD says, Israel is my son, my firstborn,
23 and I have said to you, “Let my son go, that he may serve me;” and you have refused to let him go. Behold, I will kill your firstborn son.’”