Musnad Ahmad 1302:10

Bahz narrated to us, Bukayr ibn Abi al-Sumayt narrated to me, Qatadah narrated to us, from Salim ibn Abi al-Ja'd al-Ghatafani, from Ma'dan ibn Abi Talhah al-Ya'mari, from Abu al-Darda', that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Is one of you unable to recite one-third of the Qur'an every day?" They said: "O Messenger of Allah, we are weaker than that and less able." He said: "For indeed, Allah divided the Qur'an into three parts, and 'Say, He is Allah, [who is] One' is one of its parts."