It is narrated from Sayyiduna Hisham bin Amir (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "(Exchanging) gold for silver is usury (riba), unless it is cash for cash, hand to hand."
Imam al-Tabarani (may Allah have mercy on him) said: "No one narrated this hadith from Ayyub (may Allah have mercy on him) except Ma'mar (may Allah have mercy on him)."
Hadith Referenceالمعجم الأوسط / باب الألف / 2305
Hadith Takhrij«أخرجه عبد الرزاق فى «مصنفه» برقم: 14545»