The Messenger of Allah ﷺ said, "Do not meet the riders to buy from them before they reach the market, do not undercut one another in sales, do not artificially inflate prices, and let no townsman sell on behalf of a desert dweller."
أخبرنا قتيبة عن مالك عن أبي الزناد عن الأعرج
أن رسول الله ﷺ قال لا تلقوا الركبان للبيع ولا يبع بعضكم على بيع بعض ولا تناجشوا ولا يبيع حاضر لباد
Bukhari narrated it in the Book of Sales, in the chapter on the prohibition against a seller withholding milk from camels, cows, and sheep, and every... (etc.), hadith 1515, and hadith of muhaffalah (animals with withheld milk), hadith 2150. Muslim also narrated it in the Book of Sales, in the chapter on the prohibition of a man outbidding his brother's sale, hadith 11, from the hadith of Malik through this same chain. It is also found in Muwatta (Yahya's recension), 2/683, 684, and in Sunan al-Kubra, hadith 6087.