Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever buys a she-camel or a goat whose milk has been held back has one of two options: if he wishes, he may return it, and with it a vessel of grain."
(b) Some narrate from Ibn Sirin (may Allah have mercy on him): "a sa' of grain, and he has the option for three days." Imam al-Bukhari (may Allah have mercy on him) said: (The narrations of) dates are more numerous.
The Shaykh (may Allah have mercy on him) said: By "ta'am" (grain) in this hadith dates are meant: he should give dates, not wheat.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10720