The Messenger of Allah ﷺ said, "Do not go out to meet caravans in order to buy from them before they reach the market. Do not undercut one another's sales. Do not engage in artificial bidding. A town-dweller must not sell on behalf of a desert-dweller. Do not tie up the udders of camels and sheep to make them appear to produce more milk. Whoever buys such an animal after that has the better of two choices once he has milked it. If he is pleased with it, he keeps it, and if he is displeased, he returns it along with a measure of dates."
أن رسول الله ﷺ قال: لا تلقوا الركبان للبيع، ولا يبع بعضكم على بيع بعض، ولا تناجشوا، ولا يبع حاضر لباد، ولا تصروا الإبل والغنم، فمن ابتاعها بعد ذلك فهو بخير النظرين بعد أن يحلبها: إن رضيها أمسكها، وان سخطها ردها وصاعا من تمر.
Malik narrated it in "Sales" (96), from Abu Zinad, from Araj, from Abu Hurairah.Bukhari narrated it in "Sales" (2150), and Muslim in "Sales" (11:1515), both of them through the route of Malik, with the same wording.