Sahih Bukhari Volume 003, Book 042, Hadith Number 614.

Narated By Ibn 'Umar : Allah's Apostle said, "An animal should not be milked without the permission of its owner. Does any of you like that somebody comes to his store and breaks his container and takes away his food? The udders of the animals are the stores of their owners where their provision is kept, so nobody should milk the animals of somebody else, without the permission of its owner."