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).


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.


Sahih Muslim, The Book of Transactions