Sahih Bukhari Volume 3, Book 38, Hadith Number 506.
Narated By Abu Said al-Khudri : Once Bilal brought Barni (i.e. a kind of dates) to the Prophet and the Prophet asked him, “From where have you brought these?” Bilal replied, “I had some inferior type of dates and exchanged two Sas of it for one Sa of Barni dates in order to give it to the Prophet; to eat.” Thereupon the Prophet said, “Beware! Beware! This is definitely Riba (usury)! This is definitely Riba (Usury)! Don’t do so, but if you want to buy (a superior kind of dates) sell the inferior dates for money and then buy the superior kind of dates with that money.”