Musnad Ahmad 30:187

Sufyan narrated to us, from Abu al-Zinad, from al-A’raj, from Abu Hurayrah, attributing it to the Prophet, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not meet the merchandise (on the way before it reaches the market), and do not tie up the udders of sheep and camels for sale. Whoever buys them after that, he has the choice of the two options: if he wishes, he may keep them, and if he wishes, he may return them along with a sa’ of dates, not wheat."