Exodus 4:21
“And 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.”
And the Lord said unto Moses in Midian, Go, return into Egypt: for all the men are dead which sought thy life.
And 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.
And 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.
And thou shalt say unto Pharaoh, Thus saith the Lord, Israel is my son, even my firstborn:
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.
Counter-Arguments
The strongest case that this verse does not belong in this theme.

Cross-References
“Therefore hath he mercy on whom he will have mercy, and whom he will he hardeneth.”
“Therefore hath he mercy on whom he will have mercy, and whom he will he hardeneth.”
“He hath blinded their eyes, and hardened their heart; that they should not see with their eyes, nor understand with their heart, and be converted, and I should heal them.”
“And he hardened Pharaoh’s heart, that he hearkened not unto them; as the Lord had said.”
“And the heart of Pharaoh was hardened, neither would he let the children of Israel go; as the Lord had spoken by Moses.”