Exodus 11:6-10

So there shall be a great cry in all the land of Egypt, such as there has not been before and such as shall never be again.

But not even a dog will threaten any of the sons of Israel, nor anything from person to animal, so that you may learn how the Lord distinguishes between Egypt and Israel.’

And all these servants of yours will come down to me and bow themselves before me, saying, ‘Go out, you and all the people who follow you,’ and after that I will go out.” And he left Pharaoh in the heat of anger.

Then the Lord said to Moses, “Pharaoh will not listen to you, so that My wonders will be multiplied in the land of Egypt.”

So Moses and Aaron performed all these wonders before Pharaoh; yet the Lord hardened Pharaoh’s heart, and he did not let the sons of Israel go out of his land.