Musnad Ahmad 3:46

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, Abdullah ibn Lahi’ah narrated to us, Musa ibn Wardan narrated to us, he said: I heard Sa’id ibn al-Musayyib saying: I heard Uthman delivering a sermon upon the pulpit, and he was saying: I used to buy dates from a clan of the Jews called Banu Qaynuqa’, and I would sell them for a profit. That reached the Messenger of Allah, peace and blessings of Allah be upon him, so he said: "O Uthman, when you buy, then measure, and when you sell, then measure."