Musannaf Ibn Abi Shayba 13:1467

Abu Bakr narrated to us, he said: Waki’ narrated to us, from al-Rabi’ ibn Nafi’, he said: We saw ourselves, and among us were the companions of the Messenger of Allah (may Allah bless him and grant him peace), when loads [of grain] were brought and cast in the prayer ground (musalla). A man would say: "I have measured such and such [amount]," but he would not sell it by measurement; rather, he would sell it by estimation (mujazafa), and they did not see any harm in that.