Sahih Muslim
The Book of Transactions (Kitab Al-Buyu)
Book Number: 010 / Hadith Number: 3906
Ibn 'Abbas (Allah be pleased with them) reported that when Allah's Prophet (may peace be upon him) came to Medina, they were paying one and two years in advance for fruits, so he said: Those who pay in advance for anything must do so for a specified weight and for a definite time.