Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "No one should milk another's animals without his permission. Would any of you like his food store to be broken into and his food scattered? The udders of their livestock likewise store their food, so no one should milk another's animals without his permission."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19643