Exodus 8:28-32

28So Pharaoh said, ‘I will let you go. You may go into the desert and offer sacrifices to the Lord your God there. But do not go very far away. And pray for me too.’

29Then Moses said, ‘I will leave you now, and I will pray to the Lord. Tomorrow, all the flies will go away from you, your officers and your people. But be careful! Do not deceive us, as you did before. You must really let the people go away to offer sacrifices to the Lord.’

30So Moses left Pharaoh and he prayed to the Lord.

31The Lord did what Moses asked him. All the flies left Pharaoh, his officers and his people. Not one fly remained!

32But again, Pharaoh's mind became hard. He refused to let the people go.