Yahya related to me from Malik that Yahya ibn Said said, "TheMessenger of Allah, mayAllah bless him and grant him peace, orderedthe two Sads to sell a vessel made of either gold or silver from thebooty. They either sold each three units of weight for four units ofweight of coins or each four units of weight for three units of weightor coins. The Messenger of Allah, may Allah bless him and grant himpeace, said to them, 'You have taken usury, so return it.' "
USC-MSA web (English) reference: Book 31, Hadith 28
Arabic reference: Book 31, Hadith 1319