Muslim bin Yasar and 'Abdullah bin 'Ubaid said: "Ubadah bin Samit and Mu'awiyah happened to meet, either in a church or in a synagogue. 'Ubadah bin Samit narrated to them and said: 'The Messenger of Allah P.B.U.H forbade us from selling silver for silver, gold for gold, wheat for wheat, barley for barley, and dates for dates:" - one of them said: "And salt for salt," but the other did not say it. - "And he commanded us to sell Wheat for barley, or barley for Wheat, hand-to-hand, however we Wished." (Sahih)