It is narrated from Sayyiduna Umar bin Khattab (may Allah be pleased with him) that he heard the Messenger of Allah (peace and blessings be upon him) say: "The exchange of gold for silver is usury, except when it is hand to hand, one hand giving and one hand taking. And if wheat is exchanged for wheat on credit, it is usury; and the exchange of barley for barley on credit is usury."
Hadith Referenceالسنن المأثورة / باب في البيوع / 210
Hadith Takhrijصحیح بخاری ، البيوع، باب ما يذكر في بيع الطعام والحكرة رقم : 2134، صحیح مسلم ، المساقاة، باب الصرف بيع الذهب بالورق نقدا، رقم : 1586 ۔