Musannaf Ibn Abi Shayba 13:2344

Abu Bakr narrated to us, he said: Isma'il bin Ibrahim narrated to us, from Ibn Abi Najih, from 'Abdullah bin Kathir, from Abu al-Minhal, from Ibn 'Abbas, who said: The Prophet, peace and blessings of Allah be upon him, arrived in Medina while the people were practicing *salam* (forward sales) for dates for one year, two years, and three years. He said: "Whoever practices *salam* in dates, let him practice *salam* in a known measure and a known weight, for a known term."