Musnad Ahmad 4:23

Abu Sa’id narrated to us, Ibn Lahi’ah narrated to us, Musa ibn Wardan informed 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 in *a’su* (measures). That reached the Prophet, may Allah bless him and grant him peace, so he said: "O ‘Uthman, when you buy, then measure, and when you sell, then measure."