John Calvin Commentary Exodus 9:1

"Then Jehovah said unto Moses, Go in unto Pharaoh, and tell him, Thus saith Jehovah, the God of the Hebrews, Let my people go, that they may serve me." — Exodus 9:1 (ASV)

Then the Lord said. No complaint or protest from Moses is related here; and it is possible that he was quiet and silent, while God foresaw what was necessary to do, and even commanded what He would have done. But since he only gives a brief summary of events, we may reasonably assume that, as the evil grew worse, he sought the remedy from time to time.

In the denunciation, “the Lord God of the Hebrews” is no meaningless repetition, so that Pharaoh might learn that He, whom Pharaoh thought to have repelled in his great pride, was still in the field against him. For God insults Pharaoh's ferocity and, by declaring His own name, contemptuously defies Pharaoh's wrath.

We have already said that Pharaoh is convicted of sacrilege, both in his oppression of God’s people and in defrauding God Himself of the honor due to Him; therefore, those words, Let my people go, that they may serve me, serve to aggravate his sin.