Musnad Ahmad 30:338

Rab'i ibn Ibrahim narrated to us, Abd al-Rahman ibn Ishaq narrated to us, from Muslim ibn Abi Muslim, who said: I saw Abu Hurairah, and we were young boys coming to the Bedouins, saying: "O Bedouin, we will sell for you." He said: "Leave him, let him sell his own merchandise." Then Abu Hurairah said: "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) forbade that a city dweller should sell for a Bedouin."