Atiyyah narrates: Sayyiduna Abu Sa'id (may Allah be pleased with him) said to Sayyiduna Ibn Abbas (may Allah be pleased with them both): Repent to Allah, the Mighty and Majestic. He said: I seek Allah's forgiveness and I repent to Him. Abu Sa'id said: Do you not know that the Messenger of Allah (peace and blessings be upon him) forbade (selling) gold for gold and silver for silver (with any difference in amount), and said: "I fear usury (riba) for you"? I asked Atiyyah: What is usury? He said: "The excess and the difference between the two."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 455