Exodus 8:26

23 And I will put a division between my people and your people; tomorrow this sign will be seen.

24 And the Lord did so; and great clouds of flies came into the house of Pharaoh and into his servants' houses, and all the land of Egypt was made waste because of the flies.

25 And Pharaoh sent for Moses and Aaron and said, Go and make your offering to your God here in the land.

26 And Moses said, It is not right to do so; for we make our offerings of that to which the Egyptians give worship; and if we do so before their eyes, certainly we will be stoned.

27 But we will go three days' journey into the waste land and make an offering to the Lord our God as he may give us orders.

28 Then Pharaoh said, I will let you go to make an offering to the Lord your God in the waste land; but do not go very far away, and make prayer for me.

29 And Moses said, When I go out from you I will make prayer to the Lord that the cloud of flies may go away from Pharaoh and from his people and from his servants tomorrow: only let Pharaoh no longer by deceit keep back the people from making their offering to the Lord.