Tafsir of Yusuf 12:73

{تالله} (By Allah): An oath containing a sense of astonishment at what was attributed to them.

They said {لقد علمتم} (You have certainly known): They appealed to their [the Egyptians'] knowledge, based on what had been established for them regarding their religious integrity and trustworthiness during their two visits and their dealings with the King.

This is because:

1. They entered with the mouths of their pack animals muzzled, so they would not consume the crops or food of anyone in the marketplace.
2. They returned the merchandise they found in their saddlebags.

{وما كنا سارقين} (And we have not been thieves): We have never been described as thieves, for it is a trait contradictory to our state.

{قالوا فما جزآؤه إن كنتم كاذبين * قالوا جزؤه من وجد فى رحله فهو جزاؤه كذالك نجزى الظالمين}

(They said: "What then shall be the penalty for it, if you are liars?" They said: "The penalty for it is that he in whose saddlebag it is found, he himself shall be the penalty for it. Thus do we punish the wrongdoers.")