Book 35: Sales in which a Price is paid for Goods to be Delivered Later (As-Salam)
Volume 3, Book 35, Number 441: 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)." Volume 3, Book 35, Number 442: Narrated Ibn Abi Najih: as above, mentioning only specific measure. Volume 3, Book 35, Number 443: Narrated 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." Volume 3, Book 35, Number 444: Narrated Ibn Abi Najih: as above, saying, &