Sahih Muslim Book 10, Hadith Number 3872.

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 Sa’id (Allah be pleased with him) reported: Dates were brought to Allah’s Messenger (may peace be upon him), and he said: These dates are not like our dates, whereupon a man said: We sold two sa’s of our dates (in order to get) one sa’, of these (fine dates), whereupon Allah’s Messenger (may peace be upon him) said: That is interest; so return (these dates of fine quality), and get your (inferior dates) ; then sell our dates (for money) and buy for us (with the help of money) such (fine dates).