Ibn 'Umar reported Allah's Messenger (ﷺ) having said this:None (of you) should milk the animal of another, but with his permission. Does any one of you like that his chamber be raided, and his vaults be broken, and his foodstuff be removed? Verily the treasures for them (those who keep animals) are the udders of the animals which feed them. So none of you should milk the animal of another but with his permission.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى التَّمِيمِيُّ، قَالَ قَرَأْتُ عَلَى مَالِكِ بْنِ أَنَسٍ عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " لاَ يَحْلُبَنَّ أَحَدٌ مَاشِيَةَ أَحَدٍ إِلاَّ بِإِذْنِهِ أَيُحِبُّ أَحَدُكُمْ أَنْ تُؤْتَى مَشْرُبَتُهُ فَتُكْسَرَ خِزَانَتُهُ فَيُنْتَقَلَ طَعَامُهُ إِنَّمَا تَخْزُنُ لَهُمْ ضُرُوعُ مَوَاشِيهِمْ أَطْعِمَتَهُمْ فَلاَ يَحْلُبَنَّ أَحَدٌ مَاشِيَةَ أَحَدٍ إِلاَّ بِإِذْنِهِ " .
Muslim
The Book of Lost Property • Hadith 13