Genesis 42:15-17

"Hereby shall ye be proved: by the life of Pharaoh ye shall not go forth hence, except your youngest brother come hither. Send one of you and let him fetch your brother, and ye shall be bound, that your words may be proved, whether there be truth in you: or else by the life of Pharaoh surely ye are spies. And he put them all together into ward three days."

So the accused must prove themselves innocent. To this point, Joseph had disposed of the case fully in keeping with what anyone in Egypt would have considered to be absolutely proper. After a period of three days, during which period practically everyone in Egypt would have forgotten all about the incident, Joseph would again review the case and reduce the number from ten to one of those who would be left in prison. Perhaps Joseph remembered the members of his father's house and thought of their dire need of food, and therefore he sent them all home except Simeon, their sacks laden with grain, and their money returned, as related at once by the sacred author.

"By the life of Pharaoh ..." This was a common oath, corresponding to an expression found in 2 Kings 2:4, "And Elisha said, As Jehovah liveth." In Egypt, Pharaoh was considered a god, but Joseph's early training had taught him the name of the true God, a truth he had not forgotten, despite his using the usual Egyptian expression here.