(a) Yahya bin Yahya from Sufyan bin Uyaynah, (b) Imam al-Shafi'i (may Allah have mercy on him) from Sufyan bin Uyaynah, (c) Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) said: When the Prophet (peace and blessings be upon him) came to Madinah, the people of Madinah used to make advance sales (salaf) in dates for two and three years. Thereupon the Messenger of Allah (peace and blessings be upon him) said: "Whoever makes an advance sale, let him do so for a known measure, a known weight and a known term."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11083