Sahih Muslim Book 10, Hadith Number 3879.

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.

Ata’ b. Abu Rabah reported: Abu Sa’id al-Khudri (Allah be pleased with them) met Ibn ‘Abbas (Allah be pleased with them) and said to him: What do you say in regard to the conversion (of commodities or money) did you hear it from Allah’s Messenger (may peace be upon him), or is it something which you found In Allah’s Book, Majestic and Glorious? Thereupon Ibn Abbas (Allah be pleated with them) said: I don’t say that. So far at Allah’s Messenger (may peace be upon him) is concerned, you know him better, and to far as the Book of Allah to concerned, I do not know it (more than you do), but ‘Usama b. Zaid (Allah be pleased with him) narrated to me Allah’s Messenger (may peace be upon him) as having said this: Beware, there can be an element of interest in credit.