Midrash Tanchuma Buber
(Exod. 7:14:) AND THE LORD SAID UNTO MOSES: PHARAOH'S HEART IS HARDENED. The Holy One said: By your life, through that word (HARDENED), because you have hardened (rt.: KBD) your heart, I will be honored (rt.: KBD).76Tanh., Exod. 2:12; Exod. R. 9:8. It is so stated (in Exod. 14:18): WHEN I HAVE BEEN HONORED (rt.: KBD) THROUGH PHARAOH.