Musnad Ahmad 3:7

Waki’ narrated to us, Sufyan narrated to us, and ‘Abd al-Rahman narrated to us, from Sufyan, from ‘Alqamah ibn Marthad, from Abu ‘Abd al-Rahman, from ‘Uthman, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The best of you is he who learned the Qur’an and taught it."