Exodus 8:15, 31-32

But when Pharaoh saw that relief had come, he became stubborn. He refused to listen to Moses and Aaron, just as the LORD had predicted.

And the LORD did as Moses asked and caused the swarms of flies to disappear from Pharaoh, his officials, and his people. Not a single fly remained. But Pharaoh again became stubborn and refused to let the people go.