Sahih Bukhari Volume 3, Book 38, Hadith Number 499.
Narated By Abu Said Al-Khudri and Abu Huraira : Allah’s Apostle employed someone as a governor at Khaibar. When the man came to Medina, he brought with him dates called Janib. The Prophet asked him, “Are all the dates of Khaibar of this kind?” The man replied, “(No), we exchange two Sa’s of bad dates for one Sa of this kind of dates (i.e. Janib), or exchange three Sa’s for two.” On that, the Prophet said, “Don’t do so, as it is a kind of usury (Riba) but sell the dates of inferior quality for money, and then buy Janib with the money”. The Prophet said the same thing about dates sold by weight. (See Hadith No. 506).