Sahih Muslim
The Book of Transactions (Kitab Al-Buyu)
Book Number: 010 / Hadith Number: 3628
Abu Huraira (Allah be pleased with him) reported it directly from Allah's Apostle (may peace be upon him): The townsman'should not sell for a man from the desert (with a view to taking advantage of his ignorance of the market conditions of the city). And Zuhair reported from the Holy Prophet (may peace be upon him) that he forbade the townsman to sell on behalf of the man from the desert.