Musnad Ahmad 1301:13

Sulayman ibn Dawud—meaning Abu Dawud al-Tayalisi—narrated to us, he said: Shu'bah narrated to us, from Qatadah, he said: I heard Salim ibn Abi al-Ja'd narrating from Ma'dan ibn Abi Talhah, from Abu al-Darda', from the Prophet, peace and blessings of Allah be upon him, that he said: "Is one of you unable to recite a third of the Qur'an in a night?" It was said: "And who is capable of that?" He said: "Recite: 'Say, He is Allah, [who is] One.'"