Sayyiduna Abu Sa'id (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, salt for salt, like for like. Whoever gives more or takes more has engaged in usury (riba)." It was said: O Messenger of Allah, the man in charge of your dates buys one sa' for two sa'. So he sent for him, and he said: O Messenger of Allah, my dates are such and such, and they will not take them unless I give them more. The Prophet (peace and blessings be upon him) said: "Do not do that."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5428