Book 35: Sales in which a Price is paid for Goods to be Delivered Later (As-Salam)
Hadith 441 (Volume 3)
Narrated Ibn Abbas:
Allah's Apostle came to Medina and the people used to pay in advance the price of fruits to be delivered within one or two years. (The sub-narrator is in doubt whether it was one to two years or two to three years.) The Prophet said, "Whoever pays money in advance for dates (to be delivered later) should pay it for known specified weight and measure (of the dates)."