Musnad Ahmad 30:2099

Ali ibn Ishaq narrated to us, he said: Abdullah narrated to us, he said: Ubaydullah ibn Umar informed us, from Sa'id al-Maqburi, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, that he: "Forbade meeting the caravans (for trade before they reach the market), and that a town-dweller should sell on behalf of a desert-dweller."