Exodus 8:25-29

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."

28So Pharaoh said, "I will let you go to sacrifice to the Lord your God in the wilderness; only you must not go very far away. Plead for me."

29Then Moses said, "Behold, I am going out from 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 let not Pharaoh cheat again by not letting the people go to sacrifice to the Lord."