Sahih Muslim 1520
Sahih Muslim · Book 21: The Book of Transactions · The prohibition of the town-dweller selling on behalf of a bedouin
Abu Huraira (Allah be pleased with him) reported it directly from Allah's Apostle (ﷺ):
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، وَعَمْرٌو النَّاقِدُ ، وَزُهَيْرُ بْنُ حَرْبٍ ، قَالُوا حَدَّثَنَا سُفْيَانُ ، عَنِ الزُّهْرِيِّ ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ ، عَنْ أَبِي هُرَيْرَةَ ، يَبْلُغُ بِهِ النَّبِيَّ صلى الله عليه وسلم قَالَ " لاَ يَبِعْ حَاضِرٌ لِبَادٍ " . وَقَالَ زُهَيْرٌ عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ نَهَى أَنْ يَبِيعَ حَاضِرٌ لِبَادٍ .
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 Prophet (ﷺ) that he forbade the townsman to sell on behalf of the man from the desert.