Musannaf Ibn Abi Shayba 13:106

Waki’ narrated to us, from Kahmas ibn al-Hasan, from Maymun al-Qannad, who said: I said to Sa’id ibn al-Musayyib: A man buys livestock, and I am looking at their weight; I buy them by their weight. He said: It used to be said that this is usury (riba) mixed with measurement and weighing.