1؎: Because if such an animal is sold, it will be a deception against the buyer, and it is not permissible to deceive anyone.
Explanation & Benefits
Hafiz Muhammad Ameen
Urdu marginal note: “If one intends to sell,” so that the buyer is not deceived. However, if there is no plan to sell and the milk is little, then it is permissible to skip a day and milk the animal, because in this there is no intention to deceive anyone. Some are of the opinion that keeping the milk collected in the udders causes discomfort to the animal, therefore the milk should be milked regularly, but this is a medical issue rather than a legal (shar‘i) one.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 4491