Musnad Ahmad 28:137

Hassan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn Abdullah narrated to us, from Abu Abd al-Rahman al-Hubuli, from Abdullah ibn Amr: That Abu Ayyub al-Ansari was in a gathering, and he was saying: "Is one of you not able to stand with a third of the Quran every night?" They said: "And is one able to do that?" He said: "For 'Say, He is Allah, [who is] One' is a third of the Quran." He said: Then the Prophet, peace and blessings of Allah be upon him, came while he was hearing Abu Ayyub, and the Messenger of Allah, peace and blessings of Allah be upon him, said: "Abu Ayyub has spoken the truth."