Musnad Abu Ya'la al-Mawsili 110:80

And from Ibn ‘Uyaynah, he said: Ibn Abi Najih narrated to us, from ‘Abdullah bin Kathir, from Abu al-Minhal, who said: I heard Ibn ‘Abbas saying: The Prophet, may Allah bless him and grant him peace, arrived in Medina while they were practicing *salam* (forward sales) in dates or fruit for two or three years. He said: "Whoever practices *salam* in dates, let him practice it in a known measure and a known weight, for a known term."