Musnad Abu Ya'la al-Mawsili 159:73

And with his chain of narration, he said: The Messenger of Allah (ﷺ) said: "Do not meet the caravans for the purpose of trade, and do not outbid one another, and let not some of you sell over the sale of others. And do not tie up the udders of camels and sheep. Whoever buys them after that, he is at the best 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, and he returns with them a sa' of dates."