Joseph: "You speak deceitful words, when you call yourselves sons of Abraham. By the life of Pharaoh, ye are spies, and you did go from one disreputable house to another that none might discover you."
The expression "by the life of Pharaoh" might have betrayed Joseph's real feeling to his brethren, had they but known his habit of taking this oath only when he meant to avoid keeping his word later.
Joseph continued to speak to his brethren: "Let us suppose you should discover your brother serving as a slave, and his master should demand a high sum for his ransom, would you pay it?"