“When Pharaoh tells you, ‘Perform a miracle,’ you are to say to Aaron, ‘Take your staff and throw it down before Pharaoh,’ and it will become a serpent.”
Pharaoh demands a miracle not to be convinced but hoping none will come, so he can cloak his unbelief. The rod becomes a serpent to strike terror into him, for God deals with sinners gradually, making them fear before they feel. Yet the magicians' imitation dulls the force of what should have broken his heart.
AI summary
Commenting on Exodus 7:8-13
When Pharaoh shall speak unto you, saying, show a miracle for you,.... To prove that they came from God, the Jehovah they said they did, and that they were his ambassadors, and came in his name, and made the demand for him; which when he seriously reflected on things, he would be ready to require, hoping they would not be able to show any, and...
When Pharaoh shall speak unto you, &c.--The king would naturally demand some evidence of their having been sent from God; and as he would expect the ministers of his own gods to do the same works, the contest, in the nature of the case, would be one of miracles.