Musannaf Ibn Abi Shayba 13:1359

Abu Bakr narrated to us, he said: Yahya ibn Abi Za’idah and Ibn Abi Ghaniyah narrated to us, from ‘Abd al-Malik ibn Abi Ghaniyah, from al-Hakam, who said: Food was brought to ‘Uthman during the time of the Prophet, may Allah bless him and grant him peace. He (the Prophet) said: "Go with us to ‘Uthman to help him sell his food." He stood by his side, and ‘Uthman was saying regarding this sack: "It is such and such, and I am selling it for such and such." The Messenger of Allah, may Allah bless him and grant him peace, said: "When you have specified (the price and quantity), then measure it out."