Musnad Ahmad 936:1

Hushaym narrated to us, from Husayn, from Hilal ibn Yasaf, from Abd al-Rahman ibn Abi Layla, from Ubayy ibn Ka'b, or from a man of the Ansar, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever recites 'Say, He is Allah, [who is] One,' it is as if he has recited one-third of the Quran."