Imam Malik's Muwatta, Chapter: General Subjects

Malik related to me from Nafi from Ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, said, "No one should milk someone else's cow without his permission. Would any of you like someone to come to his apartment, break into his larder, and take his food? The udders of cows guard their food for their owners, so no one should milk someone else's cow without his permission."