Sahih Bukhari Volume 3, Book 34, Hadith 2159

حَدَّثَنَا الصَّلْتُ بْنُ مُحَمَّدٍ، حَدَّثَنَا عَبْدُ الْوَاحِدِ، حَدَّثَنَا مَعْمَرٌ، عَنْ عَبْدِ اللَّهِ بْنِ طَاوُسٍ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ لاَ تَلَقَّوُا الرُّكْبَانَ وَلاَ يَبِيعُ حَاضِرٌ لِبَادٍ ‏"‏‏.‏ قَالَ فَقُلْتُ لاِبْنِ عَبَّاسٍ مَا قَوْلُهُ لاَ يَبِيعُ حَاضِرٌ لِبَادٍ قَالَ لاَ يَكُونُ لَهُ سِمْسَارًا‏.‏

Narrated By Tawus : Ibn 'Abbas said, "Allah's Apostle said, 'Do not go to meet the caravans on the way (for buying their goods without letting them know the market price); a town dweller should not sell the goods of a desert dweller on behalf of the latter.' I asked Ibn 'Abbas, 'What does he mean by not selling the goods of a desert dweller by a town dweller?' He said, 'He should not become his broker.'"