Exodus 8:28-32

28And Pharaoh said, I will let you go, that ye may sacrifice to the Lord your God in the wilderness; only ye shall not go very far away: intreat for me.

29And Moses said, Behold, I go out from thee, and I will intreat the Lord that the swarms of flies may depart from Pharaoh, from his servants, and from his people, to morrow: but let not Pharaoh deal deceitfully any more in not letting the people go to sacrifice to the Lord.

30And Moses went out from Pharaoh, and intreated the Lord.

31And the Lord did according to the word of Moses; and he removed the swarms of flies from Pharaoh, from his servants, and from his people; there remained not one.

32And Pharaoh hardened his heart at this time also, neither would he let the people go.