Musnad Ahmad 31:316

I read to Abd al-Rahman: Malik, from Abd al-Rahman ibn Abd Allah ibn Abd al-Rahman ibn Abi Sa'sa'ah, from his father, from Abu Sa'id al-Khudri, that a man heard a man reciting: "Say, He is Allah, [who is] One," repeating it during the night. When morning came, he went to the Messenger of Allah, peace and blessings of Allah be upon him, and mentioned that to him. The man seemed to consider it insignificant. The Messenger of Allah, peace and blessings of Allah be upon him, said: "By the One in whose hand is my soul, it is equivalent to one-third of the Quran."