Exodus 8:15, 19, 32

But when Pharaoh saw that there was respite, he hardened his heart, and hearkened not unto them; as the LORD had said.

Then the magicians said unto Pharaoh, This is the finger of God: and Pharaoh's heart was hardened, and he hearkened not unto them; as the LORD had said.

And Pharaoh hardened his heart at this time also, neither would he let the people go.