Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) forbade milking livestock without the permission of their owners, and he said: "Would any of you like someone to come to his storeroom, in which his food is kept, and carry off what is in it? The udders of their livestock are like what is in their storerooms."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19644