Musannaf Ibn Abi Shayba 13:105

Marwan ibn Mu'awiyah narrated to us, from Ziyad, the freed slave of the family of Sa'id, who said: I said to Sa'id ibn al-Musayyib: A man bought food and measured it; is it permissible for me to buy it by the measurement of the [first] man? He said: No, not until it is measured in your presence.