Exodus 8:27

We will go three days' journey into the wilderness
As was first insisted on, and from which demand they should not depart:

and sacrifice unto the Lord our God, as he shall command us;
both what sacrifices shall be offered, and the manner in which they shall be done, both which seemed for the present in a good measure undetermined and unknown; and therefore it was possible, and very probable, that in one or the other they should give offence to the Egyptians, should they sacrifice among them, being at all events resolved to do as the Lord should command them.