Musnad Ahmad 30:2431

Yahya narrated to us, from Shu'bah, who said: Muhammad ibn Ziyad narrated to us, 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) and returns it, let him return with it a *sa'* of dates, not wheat."