Sahih Muslim Book 10, Hadith Number 3869.
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 Huraira and Abu Sa’id al-Khudri (Allah be pleased with them) reported that Allah’s Messenger (may peace be upon him) deputed a person from Banu ‘Adi al-Ansari to collect revenue from Khaibar. He came with a fine quality of dates, whereupon Allah’s Messenger (may peace be upon him) said to him: Are all the dates of Khaibar like this? He said: Allah’s Messenger, it is not so. We buy one sa’ of (fine quality of dates) for two sa’s out of total output (including even the inferior quality of dates), whereupon Allah’s Messenger (may peace be upon him) said: Don’t do that, but like for like, or sell this (the inferior quality and receive the price) and then buy with the price of that, and that would make up the measure.