Sahih Bukhari | Volume 3 | Hadith Number 294
Narrated by Abu Said
We used to be given mixed dates (from the booty) and used to sell (barter) two Sas of those dates) for one Sa (of good dates). The Prophet said (to us), "No (bartering of) two Sas for one Sa nor two Dirhams for one Dirham is permissible", (as that is a kind of usury). (See Hadith No. 405).