Exodus 8:29
And Moses said, Behold, I go out from thee, and I will intreat the LORD that the swarms of flies may depart from Pharaoh, from his servants, and from his people, to morrow: but let not Pharaoh deal deceitfully any more in not letting the people go to sacrifice to the LORD.
Pharaoh's consent to let Israel go into the wilderness to sacrifice, but not very far away, together with his request to be prayed for, was given only under chastening and the pressure of the plague. In answer Moses promised to go out from his presence and entreat the LORD that the swarms of flies would depart from Pharaoh, from his servants, and from his people by the next day, by tomorrow. At the same time he warned Pharaoh not to deal deceitfully any more and not to break his promise again by refusing to let the people go to sacrifice to the LORD.
The sacrifice could not be made in the land of Egypt, for Israel would sacrifice beasts which the Egyptians worshiped as sacred. To kill one of these sacred animals, even accidentally, was counted an unpardonable offense answered by death, and would cause the Egyptians to stone Israel. Therefore Israel must go three days' journey into the wilderness and sacrifice there as God had commanded, as Moses had proposed.
Moses then entreated the LORD, and the LORD removed the plague so completely that not one fly remained. Yet when relief came Pharaoh hardened his heart and still refused to yield and let the people go. The plague of flies, coming and going at Moses' command, exposed the impotence of Beelzebub, worshiped as the Egyptians' protector from such swarms, showing it had no power to protect them.