Exodus 9:35
And the heart of Pharaoh was hardened, neither would he let the children of Israel go; as the LORD had spoken by Moses. And the heart of Pharaoh was hardened, neither would he let the children of Israel go; as the LORD had spoken by Moses.
Exodus 9:35 describes the outcome of the plague of hail mingled with fire, accompanied by lightnings and thunder. The time of that plague was told beforehand, so it could not be dismissed as chance. It demonstrated to the Egyptians that the whole earth is under the command of the God of the Hebrews, and that thunder, hail, and storm obey His voice. During the storm Pharaoh, the proud king who had defied God by asking who is the Lord that I should obey His voice, humbled himself and admitted I have sinned, declared the Lord righteous and himself and his people wicked, and begged for intercession that the thunder and lightning might cease.
His confession and promise had not changed his heart, but were wrung from him by terror and anguish. His acknowledgment was prompted by a desire to escape further punishment, not by sorrow for sin or a changed purpose, and so did not amount to genuine repentance. Once the hail ceased and his fear passed, he returned to stubbornness and defiance of God, his heart hardened, and he would not let the children of Israel go. This refusal was exactly as the Lord had foretold through Moses.