The Messenger of Allah ﷺ said, "Do not meet the caravans to buy from them before they reach the market, do not undercut one another in sales, and do not tie up the udders of camels and sheep to make them appear to give more milk. Whoever buys such an animal afterward has the better of two choices once he milks it. If he is pleased with it, he may keep it, and if he is displeased, he may return it along with a saa of dates."
حدثنا عبد الله بن مسلمة عن مالك عن أبي الزناد عن الأعرج
أن رسول الله ﷺ قال لا تلقوا الركبان للبيع ولا يبع بعضكم على بيع بعض ولا تصروا الإبل والغنم فمن ابتاعها بعد ذلك فهو بخير النظرين بعد أن يحلبها فإن رضيها أمسكها وإن سخطها ردها وصاعا من تمر
Bukhari narrated it in the Book of Sales, in the chapter on the prohibition against the seller withholding milk in the udders of camels and cows and so on, hadith no. 2150, and Muslim in the Book of Sales, in the chapter on the prohibition of a man outbidding another's sale and so on, hadith no. 11/1515, from the hadith of Malik, with this wording. It is also found in the Muwatta (narrated by Yahya) 2/683, 684.