Musnad Ahmad 30:2830

'Abd al-Rahman narrated to us, he said: Dawud ibn Qays narrated to us, from Musa ibn Yasar, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever buys a *musarrah* (an animal whose milk has been withheld in the udder to deceive the buyer) ewe, let him milk it. If he is not pleased with it, let him return it, and let him return with it a *sa'* of dates."