Sahih Bukhari
Sales and Trade
Volume: 003 / Book Number: 034 / Hadith Number: 359
Narrated '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).