It is narrated from Sayyiduna Anas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Do not engage in mulamasah sales (finalising a deal merely by touching), do not artificially raise the price to deceive the buyer, do not engage in sales involving deception and uncertainty (gharar sales), and let no town-dweller take hold of the goods of a desert-dweller (in order to sell them). Whoever buys an animal whose milk has been held back in its udder (to deceive the buyer), let him milk it for three days; then if he (on learning the truth) wants to return it, let him return it with a sa' of dates."
Hadith Referenceمسند أبي يعلى الموصلي / مسند أنس بن مالك / 2767