Sahih Muslim Book 10, Hadith Number 3630.

Chapter : It is forbidden for the townsman to sell on behalf of the man of the desert.

Jabir (Allah be pleased with him) reported Allah’s Messenger (may peace be upon him) as saying: The townsman should not sell for a man from the desert, leave the people alone, Allah will give them provision from one another. Yahya reported it with a slight change of words.