Sahih Bukhari Volume 003, Book 035, Hadith Number 443.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 35. Sales in Which a Price is paid for Goods to be Delivered later (As-Salam)
Narated By Ibn 'Abbas : The Prophet came to Medina and the people used to pay in advance the price of dates to be delivered within two or three years. He said (to them), "Whoever pays in advance the price of a thing to be delivered later should pay it for a specified measure at specified weight for a specified period."