Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated that the Prophet (peace and blessings be upon him) came to Madinah while the people were paying in advance (salaf) for fruit for two years and three years. He (peace and blessings be upon him) said: "Whoever makes an advance payment (salaf), let him do so for a known measure, a known weight and a known term."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11217