Exodus 4:23
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, thy firstborn.”
Context
20And Moses took his wife and his sons, and set them upon an ass, and he returned to the land of Egypt: and Moses took the rod of God in his hand.
21And the LORD said unto Moses, When thou goest to return into Egypt, see that thou do all those wonders before Pharaoh, which I have put in thine hand: but I will harden his heart, that he shall not let the people go.
22And thou shalt say unto Pharaoh, Thus saith the LORD, Israel my son, my firstborn:
23And 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, thy firstborn.
24And it came to pass by the way in the inn, that the LORD met him, and sought to kill him.
25Then Zipporah took a sharp stone, and cut off the foreskin of her son, and cast at his feet, and said, Surely a bloody husband thou to me.
26So he let him go: then she said, A bloody husband because of the circumcision.