Musnad Ahmad 27:58

Isma'il narrated to us, Ayyub narrated to us, from Nafi', from Ibn 'Umar, from the Prophet (peace be upon him), who said: "Beware, let no one milk the livestock of a man except with his permission. Would any of you like it if someone came to his storage room, broke its door, and then took what was inside it? For indeed, in the udders of their livestock is the food of one of them. Beware, let no one milk the livestock of a man except with his permission," or he said: "by his command."