וַֽיֶּחֱזַק֙ לֵ֣ב פַּרְעֹ֔ה וְלֹ֥א שִׁלַּ֖ח אֶת־בְּנֵ֣י יִשְׂרָאֵ֑ל כַּאֲשֶׁ֛ר דִּבֶּ֥ר יְהוָ֖ה בְּיַד־מֹשֶֽׁה׃ (פ)
And the heart of Pharaoh was hardened, and he did not let the children of Israel go; as the LORD had spoken by Moses.
Read kabbalistic (mystical) interpretation of Exodus 9:35: and classical mystical sources.