Sahih Bukhari Volume 3, Book 34, Hadith Number 405.
Narated By Abu Said Al-Khudri and Abu Huraira : Allah’s Apostle appointed somebody as a governor of Khaibar. That governor brought to him an excellent kind of dates (from Khaibar). The Prophet asked, “Are all the dates of Khaibar like this?” He replied, “By Allah, no, O Allah’s Apostle! But we barter one Sa of this (type of dates) for two Sas of dates of ours and two Sas of it for three of ours.” Allah’s Apostle said, “Do not do so (as that is a kind of usury) but sell the mixed dates (of inferior quality) for money, and then buy good dates with that money.”