Musnad Ahmad 1012:64

Isma'il narrated to us, Salamah ibn 'Alqamah narrated to us, from Ibn Sirin, Muslim ibn Yasar and 'Abdullah ibn 'Ubayd—who used to be called Ibn Hurmuz—narrated to us, saying: The house brought together 'Ubadah ibn al-Samit and Mu'awiyah, either in a church or in a synagogue. 'Ubadah stood up and said: "The Messenger of Allah, may Allah bless him and grant him peace, forbade us from [exchanging] gold for gold, silver for silver, dates for dates, wheat for wheat, and barley for barley." One of them said: "And salt for salt," while the other did not say it. One of them said: "Whoever gives more or takes more has committed usury (riba)," while the other did not say it. "And he commanded us to sell gold for silver, silver for gold, wheat for barley, and barley for wheat, hand-to-hand, however we wish."