Abu Huraira (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: "Whoever buys a Musarrat (an animal whose milk has been retained in its udder), he has the option for three days; 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 wheat." Wahb says: By "Samra" is meant wheat.
Hadith Referenceالمنتقى ابن الجارود / كتاب البيوع والتجارات / 566