كتاب البيوع
21  
The Book of Transactions
(6)
Chapter: The prohibition of the town-dweller selling on behalf of a bedouin
(6)
باب تَحْرِيمِ بَيْعِ الْحَاضِرِ لِلْبَادِي ‏‏
Sahih Muslim 1521

Ibn 'Abbas (Allah be pleased with them) reported Allah's Messenger (ﷺ) as saying:

The riders (carrying merchandise) should not be met in the way, and townsman should not sell for a man of the desert. The narrator reported. I said to Ibn 'Abbas: What do these words really imply-" The townsman for the man of the desert"? He said: That he should work as a broker on his behalf.
وَحَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، وَعَبْدُ بْنُ حُمَيْدٍ، قَالاَ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنِ ابْنِ طَاوُسٍ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ، قَالَ نَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ تُتَلَقَّى الرُّكْبَانُ وَأَنْ يَبِيعَ حَاضِرٌ لِبَادٍ ‏.‏ قَالَ فَقُلْتُ لاِبْنِ عَبَّاسٍ مَا قَوْلُهُ حَاضِرٌ لِبَادٍ قَالَ لاَ يَكُنْ لَهُ سِمْسَارًا.
Reference : Sahih Muslim 1521
In-book reference : Book 21, Hadith 25
USC-MSA web (English) reference : Book 10, Hadith 3629
  (deprecated numbering scheme)