Exodus 9:34
"And when Pharaoh saw that the rain and the hail and the thunders were ceased, he sinned yet more, and hardened his heart, he and his servants."
Cross References (ranked by relevance)
And the LORD said unto Moses, Pharaoh’s heart is hardened, he refuseth to let the people go.
And he also rebelled against king Nebuchadnezzar, who had made him swear by God: but he stiffened his neck, and hardened his heart from turning unto the LORD God of Israel.
Because sentence against an evil work is not executed speedily, therefore the heart of the sons of men is fully set in them to do evil.
And humbled not himself before the LORD, as Manasseh his father had humbled himself; but Amon trespassed more and more.
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 in the time of his distress did he trespass yet more against the LORD: this is that king Ahaz.
But when Pharaoh saw that there was respite, he hardened his heart, and hearkened not unto them; as the LORD had said.