Sahih Bukhari Volume 3, Book 34, Hadith 2202

حَدَّثَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنْ عَبْدِ الْمَجِيدِ بْنِ سُهَيْلِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، وَعَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنهما ـ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم اسْتَعْمَلَ رَجُلاً عَلَى خَيْبَرَ، فَجَاءَهُ بِتَمْرٍ جَنِيبٍ، فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ أَكُلُّ تَمْرِ خَيْبَرَ هَكَذَا ‏"‏‏.‏ قَالَ لاَ وَاللَّهِ يَا رَسُولَ اللَّهِ، إِنَّا لَنَأْخُذُ الصَّاعَ مِنْ هَذَا بِالصَّاعَيْنِ، وَالصَّاعَيْنِ بِالثَّلاَثَةِ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ لاَ تَفْعَلْ، بِعِ الْجَمْعَ بِالدَّرَاهِمِ، ثُمَّ ابْتَعْ بِالدَّرَاهِمِ جَنِيبًا ‏"‏‏.‏

Narrated 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."