Musnad Ahmad 31:193

Yahya narrated to us, from Malik, [who] narrated to us, 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, from the Prophet, peace and blessings of Allah be upon him, who said: " 'Say, He is Allah, [who is] One' is equivalent—or [he said] is equal—to one-third of the Qur'an."