Exodus 8:29-33

And 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.

And Moses went out from Pharaoh, and intreated the LORD.

And 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.

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