Sahih Bukhari Volume 9, Book 93, Hadith Number 471.

Narrated By Abu Said Al-Khudri : A man heard another man reciting (in the prayers): ‘Say (O Muhammad): “He is Allah, the One.” (112.1) And he recited it repeatedly. When it was morning, he went to the Prophet and informed him about that as if he considered that the recitation of that Sura by itself was not enough. Allah’s Apostle said, “By Him in Whose Hand my life is, it is equal to one-third of the Quran.”