Sahih Muslim Book 10, Hadith Number 3876.

Chapter : He who took something as loan and made its payment back, over and above that (19 approved) and best among you is one who is best in making payment.

Abu Salih reported: I heard Abu Sa’id al-Khudri (Allah be pleased with him) said: Dinar (gold) for gold and dirham for dirham can be (exchanged) with equal for equal; but he who gives more or demands more in fact deals in interest. I said to him: Ibn ‘Abbas (Allah be pleased with them) says otherwise, whereupon he said: I met Ibn ‘Abbas (Allah be pleased with them) and said: Do you see what you say; have you heard it from Allah’s Messenger (may peace be upon him), or found it in the Book of Allah, the Glorious and Majestic? He said: I did not hear it from Allah’s Messenger (may peace be upon him) and I did not find it in the Book of Allah (Glorious and Majestic), but Usama b. Zaid narrated it to me that Allah’s Apostle (may peace be upon him) said: There can be an element of interest in credit.