John Calvin Commentary Exodus 9:34

"And when Pharaoh saw that the rain and the hail and the thunders were ceased, he sinned yet more, and hardened his heart, he and his servants." — Exodus 9:34 (ASV)

And when Pharaoh saw. Again, as usual, Pharaoh gathers audacity from the mitigation of his punishment, as security arms the reprobate against God. For as soon as the scourges of God rest for a while, they cherish the presumption that they will be unpunished and construe the short truce into an abiding peace.

Pharaoh, then, hardens anew his heart, which he seemed to have somewhat changed, as soon as he is delivered from this infliction; as though he had not been warned that others remained behind, indeed, that the hand of God was already stretched out against him.

Therefore, at the end of the chapter, Moses amplifies the crime when he adds that this had been foretold¹¹⁵ “by the hand of Moses.” We have already seen at times that the wicked king was hardened, as God had said to Moses; now, more! is expressed, namely, that Moses had been the proclaimer of his indomitable and desperate obstinacy.

¹¹⁵ Ver. 35, , A.V., marg. ref.., marg. ref.