(Read Exodus 8:20-32)
Pharaoh was early at his false devotions to the river; and shall we be for more sleep and more slumber, when any service to the Lord is to be done? The Egyptians and the Hebrews were to be marked in the plague of flies. The Lord knows them that are his, and will make it appear, perhaps in this world, certainly in the other, that he has set them apart for himself. Pharaoh unwillingly entered into a treaty with Moses and Aaron. He is content they should sacrifice to their God, provided they would do it in the land of Egypt. But it would be an abomination to God, should they offer the Egyptian sacrifices; and it would be an abomination to the Egyptians, should they offer to God the objects of the worship of the Egyptians, namely, their calves or oxen. Those who would offer acceptable sacrifice to God, must separate themselves from the wicked and profane…..
For a more detailed explanation select the link below.
Exodus 8 Bible Commentary – Matthew Henry (concise) (christianity.com)
Exodus 8:32
32 Pharaoh hardened his heart this time also, and he didn’t let the people go.