It is narrated from Sayyiduna Umar Farooq (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Selling and buying gold for gold and silver for silver is usury, except if it is done on the spot; trading wheat for wheat is usury, except if it is done on the spot; trading barley for barley is usury, except if it is done on the spot."
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 162