Exodus 8:28-32

28Pharaoh answered, “I will let you go and sacrifice to the LORD your God in the wilderness, but you must not go very far. Now pray for me.”

29“As soon as I leave you,” Moses said, “I will pray to the LORD, so that tomorrow the swarms of flies will depart from Pharaoh and his officials and his people. But Pharaoh must not act deceitfully again by refusing to let the people go and sacrifice to the LORD.”

30Then Moses left Pharaoh and prayed to the LORD, 31and the LORD did as Moses requested. He removed the swarms of flies from Pharaoh and his officials and his people; not one fly remained. 32But Pharaoh hardened his heart this time as well, and he would not let the people go.