Musnad Ahmad 3:102

Yahya ibn Sa'id narrated to us, from Sufyan and Shu'bah, from Alqamah ibn Marthad, from Sa'd ibn Ubaydah, from Abu Abd al-Rahman, from Uthman, from the Prophet, peace be upon him. Sufyan said: "The best of you," and Shu'bah said: "The best of you is he who learns the Quran and teaches it."