Musnad Ahmad 30:3194

Isma'il and Yazid narrated to us, they said: Hisham narrated to us, from Muhammad ibn Sirin, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not meet the caravans (for trade before they reach the market). Whoever meets any of them and buys something from them, then the owner (of the goods) has the choice (to annul the sale) when he reaches the market."