Musnad Abu Ya'la al-Mawsili 159:97

And with his chain of narration, that the Messenger of Allah (may Allah bless him and grant him peace) 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 or sheep. Whoever buys them after that, he is at the best of the two options: if he milks them and is pleased with them, he keeps them; and if he is displeased with them, he returns them along with a Sa' of dates."