Sahih Bukhari
Lost Things Picked up by Someone (Luqaata)
Volume: 003 / Book Number: 042 / Hadith Number: 614
Narrated 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."