Musnad Abu Ya'la al-Mawsili 6:126

Narrated to us 'Abd al-Wahid, narrated to us al-Harith ibn Nabhan, narrated to us 'Asim ibn Bahdalah, from Mus'ab ibn Sa'd, from his father, 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." He said: And he took my hand and seated me in this seat of mine to teach.