John Calvin Commentary Exodus 8:31

"And Jehovah did according to the word of Moses; and he removed the swarms of flies from Pharaoh, from his servants, and from his people; there remained not one." — Exodus 8:31 (ASV)

And the Lord did according to the word of Moses. The phrase “the word” here may be interpreted as either the answer or the prayer of Moses. I prefer the former interpretation, namely, that by the outcome God proved He ratified what Moses had said, whom He had made the proclaimer of His judgment; but if anyone prefers to refer it to his prayer, let him retain his opinion. When it is added that the heart of the king was hardened at this time also, this aggravates the crime of his obstinacy, since there was no bound to his rebellion despite such a series of punishments, by which even an iron heart should have been corrected.