Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1456
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.