Musnad Ahmad 30:2143

'Affan narrated to us, he said: Hammad narrated to us, from Muhammad ibn Ziyad, he said: I heard Abu Hurairah saying: I heard Abu al-Qasim saying: "The dumb beast is a liability, the well is a liability, and the mine is a liability; and in the buried treasure is one-fifth." And whoever buys a sheep and finds it to have been withheld of its milk (musarrah), he has the choice: if he wishes, he may return it along with a sa' of dates.