Sahih Bukhari Volume 3, Book 34, Hadith Number 359.

Narated By ‘Abdullah bin Mas’ud : Whoever buys a sheep which has not been milked for a long time, has the option of returning it along with one Sa of dates; and the Prophet forbade going to meet the seller on the way (as he has no knowledge of the market price and he may sell his goods at a low price).