Musnad Ahmad 25:1511

'Abd al-Rahman ibn Mahdi narrated to us, Sufyan narrated to us, from Ibn Abi Najih, from 'Abdullah ibn Kathir, from Abu al-Minhal, from Ibn 'Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) arrived while they were practicing *salam* (forward sales) in fruits for one year and two years, or two years and three years. The Messenger of Allah (peace and blessings of Allah be upon him) said: "Practice *salam* in fruits for a known measure, a known weight, and a known time."