It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) forbade milking the animals of people and using it for oneself without their permission.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5196
Hadith Gradingحکم دارالسلام:إسناده صحيح ، خ : 2435 ، م : 1726 .