Exodus 8:24-27

24And the Lord did so. There came great swarms of flies into the house of Pharaoh and into his servants' houses. Throughout all the land of Egypt the land was ruined by the swarms of flies.

25Then Pharaoh called Moses and Aaron and said, "Go, sacrifice to your God within the land."

26But Moses said, "It would not be right to do so, for the offerings we shall sacrifice to the Lord our God are an abomination to the Egyptians. If we sacrifice offerings abominable to the Egyptians before their eyes, will they not stone us?

27We must go three days' journey into the wilderness and sacrifice to the Lord our God as he tells us."