Abu Huraira (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Whoever buys a Musarrat (an animal whose udder has been withheld from milking), then he has a choice: if he wishes to keep it, he may keep it, and if he wishes to return it, he may return it along with a sa’ of dates, not of wheat."
Hadith Referenceالمنتقى ابن الجارود / كتاب البيوع والتجارات / 565