שמות ז:יג פירוש: רשב״ם וחזקוני

ויחזק לב פרעה, he claimed that Aaron had also merely performed a better kind of sorcery.

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.

'כאשר דבר ה, “just as Hashem had said.” (when He had said: “Pharaoh will not pay heed to you.” Verse 4)