Sahih Bukhari
Virtues of the Quran
Volume: 006 / Book Number: 061 / Hadith Number: 533
Narrated Abu Said Al-Khudri:
A man heard another man reciting (Surat-Al-Ikhlas) 'Say He is Allah, (the) One.' (112. 1) repeatedly. The next morning he came to Allah's Apostle and informed him about it as if he thought that it was not enough to recite. On that Allah's Apostle said, "By Him in Whose Hand my life is, this Surah is equal to one-third of the Qur'an!"
Narrated Abu Said Al-Khudri: My brother, Qatada bin An-Nau'man said, "A man performed the night prayer late at night in the lifetime of the Prophet and he read: 'Say: He is Allah, (the) One,' (112.1) and read nothing besides that. The next morning a man went to the Prophet ,~ and told him about that . (The Prophet replied the same as (in Hadith 532) above.)