וַיֶּחֱזַק֙ לֵ֣ב פַּרְעֹ֔ה וְלֹ֥א שָׁמַ֖ע אֲלֵהֶ֑ם כַּאֲשֶׁ֖ר דִּבֶּ֥ר יְהוָֽה׃ (פ)
And Pharaoh’s heart was hardened, and he hearkened not unto them; as the LORD had spoken.
Rashbam on Exodus
ויחזק לב פרעה, he claimed that Aaron had also merely performed a better kind of sorcery.
Chizkuni
ויחזק לב פרעה, “Pharaoh’s heart remained obstinate;” seeing that his sorcerers had been able to duplicate Aaron’s feat, he concluded that Aaron was another sorcerer.
Chizkuni
'כאשר דבר ה, “just as Hashem had said.” (when He had said: “Pharaoh will not pay heed to you.” Verse 4)