Musannaf Ibn Abi Shayba 13:1464

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Fitr, that he asked al-Sha’bi about a group of Bedouins who come to us with food, so we buy from them by measure, then we say to them: "Sell to us by estimation (without measure)." He said: "No, not until you have abandoned the (first) sale."