Exodus 8:11

והכבד את לבו HE ALLOWED HIS HEART TO BE HARDENED — The word הַכְבֵּד expresses doing something (the Hebrew infinitive but having a past meaning) just as, (Genesis 12:9) “going (הלוך) and journeying (ונסוע)”. So, too, (2 Kings 3:24) “and smiting (והכת) Moab”; (I Samuel 22:13) “and enquiring (ושאול) of God for him”; (1 Kings 20:37); “smiting (הכה) and wounding (ופצוע)”. ‎ כאשר דבר ה׳ AS THE LORD HAD SPOKEN — And where had He said this? When He said, (Exodus 7:4) “Pharaoh will not hearken to you”.

וַיַּ֣רְא פַּרְעֹ֗ה כִּ֤י הָֽיְתָה֙ הָֽרְוָחָ֔ה וְהַכְבֵּד֙ אֶת־לִבּ֔וֹ וְלֹ֥א שָׁמַ֖ע אֲלֵהֶ֑ם כַּאֲשֶׁ֖ר דִּבֶּ֥ר יְהֹוָֽה׃ {ס}        

But when Par῾o saw that there was respite, he hardened his heart, and hearkened not to them; as the Lord had said.