וַיֶּחֱזַק֙ לֵ֣ב פַּרְעֹ֔ה וְלֹ֥א שָׁמַ֖ע אֲלֵהֶ֑ם כַּאֲשֶׁ֖ר דִּבֶּ֥ר יְהוָֽה׃ (פ)
И фараон'сердце его ожесточилось, и он не послушал их; как говорил Господь
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.
Ibn Ezra
Pharaoh’s heart remained hard. He himself caused the hardening of his heart, because he saw the sorcerers could replicate what Aharon did.
Rashbam
ויחזק לב פרעה, he claimed that Aaron had also merely performed a better kind of sorcery.