(2) باب السَّلَمِ فِي وَزْنٍ مَعْلُومٍ
Narrated Ibn `Abbas:
The Prophet (ﷺ) 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."
Reference: Sahih al-Bukhari 2240
In-book reference: Book 35, Hadith 3
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 35, Hadith 443