Sahih Bukhari, Chapter: Sales and Trade

Allah's Apostle said, "Do not go forward to meet the caravan (to buy from it on the way before it reaches the town). And do not urge buyers to cancel their purchases to sell them (your own goods) yourselves, and do not practice Najsh. A town dweller should not sell the goods for the desert dweller. Do not leave sheep unmilked for a long time, when they are on sale, and whoever buys such an animal has the option of returning it, after milking it, along with a Sa of dates or keeping it. it has been kept unmilked for a long period by the seller (to deceive others).