From Sahih Bukhari (Hadith 2151):
Narrated Abu Huraira:Allah's Messenger (ﷺ) said, "Whoever buys a sheep which has been kept unmilked for a long period, and milks it, can keep it if he is satisfied, and if he is not satisfied, he can return it, but he should pay one Sa of dates for the milk
Reference: Bukhari 2151