Musnad Ahmad 25:31

Isma'il ibn Ibrahim narrated to me, Ibn Abi Najih narrated to us, 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 in Medina, and the people were practicing *salam* (forward sales) for dates for one year and two years—or he said: two years and three years. He said: "Whoever practices *salam* in dates, let him practice *salam* with a known measure and a known weight."