John Gill Commentary Exodus 9:12

"And Jehovah hardened the heart of Pharaoh, and he hearkened not unto them, as Jehovah had spoken unto Moses." — Exodus 9:12 (ASV)

And the Lord hardened the heart of Pharaoh He having often, and so long hardened his own heart, God gave him up to judicial hardness of heart, to his own corruptions, the temptations of Satan, and the lying magicians about him, to make an ill use of everything that offered to him, and put a wrong construction on all that befell him, so that whatever was said to him, or inflicted on him, made no impression to any purpose: and he hearkened not unto them to Moses and Aaron, and to the Lord by them: as the Lord had spoken to Moses both that he would harden his heart, and he should not hearken to them; all this was no other than what the Lord had said should be, (Exodus 4:21) (7:4) .