וַיְחַזֵּ֤ק יְהוָה֙ אֶת־לֵ֣ב פַּרְעֹ֔ה וְלֹ֥א שָׁמַ֖ע אֲלֵהֶ֑ם כַּאֲשֶׁ֛ר דִּבֶּ֥ר יְהוָ֖ה אֶל־מֹשֶֽׁה׃ (ס)
And the LORD hardened the heart of Pharaoh, and he hearkened not unto them; as the LORD had spoken unto Moses.
Read halakhic rulings (Jewish law) on Exodus 9:12: and classical halakhic sources.