Musannaf Ibn Abi Shayba 22:161

Ahmad ibn Ishaq narrated to us, from Abd al-Wahid ibn Ziyad, from Abd al-Rahman ibn Ishaq, from al-Nu'man ibn Sa'd, from Ali, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The best of you is he who learns the Quran and teaches it."