Sunnah.com reference : Book 7, Hadith 32English translation : Book 7, Hadith 810Arabic reference : Book 7, Hadith 807
Narrated Tawus from Ibn 'Abbas (RA), Allah's Messenger (ﷺ) said:
"Do not go out to meet riders (to conduct business with them), and a city-dweller must not sell for a man from the desert." I asked Ibn 'Abbas (RA), "What did he mean by 'A city-dweller must not sell for a man from the desert.'" He replied, "He should not act as a broker for him."

[Agreed upon and the wording is al-Bukhari's].
وَعَنْ طَاوُسٍ, عَنِ اِبْنِ عَبَّاسٍ ‏-رَضِيَ اَللَّهُ عَنْهُمَا‏- قَالَ: قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ لَا تَلَقَّوْا اَلرُّكْبَانَ, وَلَا يَبِيعُ حَاضِرٌ لِبَادٍ ".‏ قُلْتُ لِابْنِ عَبَّاسٍ: مَا قَوْلُهُ: " وَلَا يَبِيعُ حَاضِرٌ لِبَادٍ? " قَالَ: لَا يَكُونُ لَهُ سِمْسَارًا }  مُتَّفَقٌ عَلَيْهِ وَاللَّفْظُ لِلْبُخَارِيِّ  1‏ .‏‏1 ‏- صحيح.‏ رواه البخاري ( 2158 )‏، ومسلم ( 1521 )‏.‏