Musnad Ahmad 30:2928

'Abd al-Rahman ibn Mahdi narrated to us, he said: Hammad narrated to us, from Muhammad ibn Ziyad, he said: I heard Abu Hurayrah saying: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "Whoever buys a sheep and finds it to be *musarrah* (one whose milk has been withheld in the udder to deceive the buyer), he has the choice. If he wishes, he may return it, and he shall return with it a *sa'* of dates."