Exodus 8:28-32

Pharaoh said, “I will let you go, so that you may sacrifice to the Lord your God in the wilderness; only you shall not go very far away. Plead for me.”

Then Moses said, “Behold, I am going to leave you, and I will plead with the Lord that the swarms of flies may depart from Pharaoh, from his servants, and from his people tomorrow; only do not let Pharaoh deal deceitfully again in not letting the people go to sacrifice to the Lord.”

So Moses left Pharaoh and pleaded with the Lord.

The Lord did as Moses asked, and removed the swarms of flies from Pharaoh, from his servants, and from his people; not one remained.

But Pharaoh hardened his heart this time also, and he did not let the people go.