It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "No one should milk another's animal without his permission. Would any of you like his storeroom to be entered, his store-chamber broken open, and his food carried away? In the same way, the udders of their livestock are a store of their food for them; their food is in them. So let no one milk another person's animal without his permission."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العارية / 11497