וַיְחַזֵּ֤ק יְהוָה֙ אֶת־לֵ֣ב פַּרְעֹ֔ה וְלֹ֥א שָׁמַ֖ע אֲלֵהֶ֑ם כַּאֲשֶׁ֛ר דִּבֶּ֥ר יְהוָ֖ה אֶל־מֹשֶֽׁה׃ (ס)
But the LORD stiffened the heart of Pharaoh, and he would not heed them, just as the LORD had told Moses.
ויחזק ה' כי לא היה סובל זה בלי ספק, כענין באיוב וגע אל עצמו ואל בשרו:
.'ויחזק ה, G’d needed to reinforce Pharaoh’s heart as without this he would not have been able to endure this plague without his resistance collapsing. We know how Satan expected Job to react to the same affliction, from Job 2,5 “if You lay a hand on his bones and flesh he will surely blaspheme.”