The Messenger of Allah ﷺ said, "Do not go out to meet the caravans on the road, do not sell against one another's sale, do not inflate prices against one another, and let no townsman sell on behalf of a desert dweller. Do not withhold the milk of camels and sheep so that their udders appear full. 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 may keep it, and if he is displeased with it, he may return it along with a measure of dates."
وبإسناده قال قال رسول الله ﷺ لا تلقوا الركبان 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 وإن سخطها ردها وصاعا من تمر
Its isnad is sahih. The imams narrated it in the chapters on sales, Bukhari 3/92, chapter on the prohibition against a seller withholding milk to inflate an animal's udder, Muslim 3/1155 no. 1515, chapter on the prohibition of a man selling over his brother's sale, Abu Dawud 3/270 no. 3443, chapter on what is forbidden regarding bargaining, Nasai 7/253 no. 4487, and Malik 2/683 no. 96.