Exodus 8:28-32

28Pharaoh said, “I will let you go to offer sacrifices to the Lord your God in the wilderness, but you must not go very far. Now pray for me.”

29Moses answered, “As soon as I leave you, I will pray to the Lord, and tomorrow the flies will leave Pharaoh and his officials and his people. Only let Pharaoh be sure that he does not act deceitfully again by not letting the people go to offer sacrifices to the Lord.”

30Then Moses left Pharaoh and prayed to the Lord,

31and the Lord did what Moses asked. The flies left Pharaoh and his officials and his people; not a fly remained.

32But this time also Pharaoh hardened his heart and would not let the people go.