Musnad Ahmad 30:707

Ibrahim ibn Khalid narrated to us, Rabah narrated to us, Ma'mar narrated to us, from Ayyub, from Ibn Sirin, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) forbade meeting the caravans (bringing goods to the market). So whoever meets them and buys (from them), then the owner (of the goods) has the choice (to cancel the sale) when he arrives at the market.