Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "When one of you buys a she-camel or a goat whose milk has been held back, after milking it he has the option to keep it or to return it, giving a sa' of dates along with it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10715