Abu Dawud Book 2, Hadith Number 1456.
Chapter : On the surah alikhlas unity of allah.
Narated By Abu Sa’id al-Khudri : A man heard another man reciting: “Say, He is Allah, One.” He was repeating it. When the next morning came, he came back to the Apostle of Allah (PBUH) and mentioned that to him. The man took it (the surah) as a small one. The Prophet (PBUH) said: By Him in Whose Hand is my life, it is equivalent to a third of Qur’an.