Musnad Ahmad 30:2874

Ishaq narrated to us, Malik narrated to me, from Abu al-Zinad, from al-A’raj, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not meet the riders (to buy their goods before they reach the market), and let not some of you outbid others, and do not practice Najash (bidding up the price without the intention to buy), and let not a city dweller sell for a bedouin, and do not tie up the udders of camels and sheep (to deceive buyers by making it appear they have much milk). Whoever buys them after that, he has the choice of the two options after he milks them: if he is pleased with them, he keeps them, and if he is displeased with them, he returns them along with a Sa’ of dates."