Musnad Abu Ya'la al-Mawsili 152:186

Muhammad ibn Abi Bakr narrated to us, al-Mu'tamir narrated to us, my father narrated to us, from Abu Uthman, from Ibn Mas'ud, who said: "Whoever buys a *muhaffalah* (an animal whose milk has been withheld in the udder to deceive the buyer) and returns it, let him return with it a *sa'* (of food)." He said: And the Prophet, may Allah bless him and grant him peace, forbade meeting the merchandise (before it reaches the market).