Exodus 9:27-35

Then Pharaoh sent for Moses and Aaron, and said to them, “I have sinned this time; the LORD is righteous, and I and my people are wicked. Pray and entreat the LORD, for there has been enough of God’s thunder and hail; I will let you go, and you shall stay here no longer.” Moses said to him, “As soon as I leave the city, I will stretch out my hands to the LORD; the thunder will cease and there will be no more hail, so that you may know [without any doubt] and acknowledge that the earth is the LORD’S. But as for you and your servants, I know that you do not yet fear the LORD God.” (Now the flax and the barley were battered and ruined [by the hail], because the barley was in the ear (ripe, but soft) and the flax was in bud, but the wheat and spelt (coarse wheat) were not battered and ruined, because they ripen late in the season.) So Moses left the city and Pharaoh, and stretched out his hands to the LORD; then the thunder and hail ceased, and rain no longer poured on the earth. But when Pharaoh saw that the rain and the hail and the thunder had ceased, he sinned again and hardened his heart, both he and his servants. Pharaoh’s heart was hardened, and he did not let the Israelites go, just as the LORD had said through Moses. [Ex 4:21]