וַיֹּ֤אמֶר יְהוָה֙ אֶל־מֹשֶׁ֔ה כָּבֵ֖ד לֵ֣ב פַּרְעֹ֑ה מֵאֵ֖ן לְשַׁלַּ֥ח הָעָֽם׃
And the LORD said unto Moses: ‘Pharaoh’s heart is stubborn, he refuseth to let the people go.
Read kabbalistic (mystical) interpretation of Exodus 7:14: and classical mystical sources.