Exodus 4:23 in the King James Version
And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn.
Context
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.’”
24 On the way at a lodging place, the LORD met Moses and wanted to kill him.