Musnad Ahmad 1301:16

Muhammad ibn Bakr and Abd al-Wahhab narrated to us, they said: Sa'id informed us, from Qatadah, from Salim ibn Abi al-Ja'd, from Ma'dan ibn Abi Talhah al-Ya'mari, from Abu al-Darda', from the Messenger of Allah, peace and blessings of Allah be upon him, who said: "Is one of you not able to recite a third of the Qur'an in a night?" They said: "We are weaker than that and less capable." He said: "Indeed, Allah, the Almighty and Majestic, divided the Qur'an into three parts, and He made 'Say, He is Allah, [who is] One' a part of the parts of the Qur'an."