Musnad Ahmad 31:402

Ishaq narrated to us, Malik narrated to us, from Abd al-Rahman ibn Abd Allah ibn Abd al-Rahman ibn Abi Sa'sa'ah al-Ansari, from his father, from Abu Sa'id al-Khudri, that a man said: "O Messenger of Allah, I have a neighbor who stands [in prayer] at night and does not recite anything except 'Say, He is Allah, [who is] One,' as if he considers it too little." The Prophet, peace and blessings of Allah be upon him, said: "By the One in whose hand is my soul, it is indeed equal to one-third of the Quran."