Sahih Muslim Book 10, Hadith Number 3873.

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: We were given to eat, during the lifetime of Allah’s Messenger (may peace be upon him), dates of different qualities mixed together, and we used to sell two sa’s of these for one sa, (of fine quality of dates). This reached Allah’s Messenger (may peace be upon him), whereupon he said: There should be no exchange of two sa’s of (inferior) dates for one sa’ (of fine dates) and two sa’s of (inferior) wheat for one sa’ of (fine) wheat and one dirham for two dirharms.