John Calvin Commentary Exodus 8:5

"And Jehovah said unto Moses, Say unto Aaron, Stretch forth thy hand with thy rod over the rivers, over the streams, and over the pools, and cause frogs to come up upon the land of Egypt." — Exodus 8:5 (ASV)

And the Lord spoke to Moses, Say to Aaron. It is questionable whether God thus instructed Moses in a continuous address, or whether He waited until Pharaoh stubbornly despised His command. It is probable, indeed, that after Pharaoh had paid no attention to the threats, the execution of the punishment was commanded.

Meanwhile, we must remember what I said before: Moses did not move even a finger but, as he had been commanded, transferred the active measures to his subordinate minister, so that Pharaoh might be treated more contemptuously. Thus, he overwhelmed the whole land, as it were, by a breath. But although in this way God cast down the fierce tyrant in his swelling pride to be trampled beneath their feet, still the wickedness of the magicians did not rest. Thus, it was necessary that the servants of God should be tested by constant struggles one after another.