“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.”
The first time that Moses made his application to Pharaoh, he produced his instructions only; now he is directed to produce his credentials, and does accordingly. 1. It is taken for granted that Pharaoh would challenge these demandants to work a miracle, that, by a performance evidently above the power of nature, they might prove their commission from the God of nature.
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.