Sahih Bukhari
Oneness, Uniqueness of Allah (Tawheed)
Volume: 009 / Book Number: 093 / Hadith Number: 471
Narrated 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."