Sahih Muslim Book 010, Hadith Number 3629.
Chapter : It is forbidden for the townsman to sell on behalf of the man of the desert.
Ibn 'Abbas (Allah be pleased with them) reported Allah's Messenger (may peace be upon him) 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.