Musnad Ahmad 30:2113

Ahmad ibn Abd al-Malik narrated to us, he said: Ubayd Allah ibn Amr narrated to us, from Ayyub, from Ibn Sirin, from Abu Hurayrah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade meeting the caravans (for trade before they reach the market). If a buyer does so, then the owner of the goods has the choice (to annul the sale) when he arrives at the market."