Sahih Muslim Book 10, Hadith Number 3877.

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.

Ubaidullah b. Abu Yazid heard Ibn ‘Abbas (Allah be pleased with them) as saying: Usama b. Zaid reported Allah’s Apostle (may peace be upon him) as saying: There can be an element of interest in credit (when the payment is not equal).