Sahih Bukhari | Volume 8 | Hadith Number 638
Narrated by Abu Sa'id Al-Khudri
A man heard another man reciting: Surat-ul-Ikhlas (The Unity) 'Say: He is Allah, the One (112) and he was repeating it. The next morning he came to Allah's Apostle and mentioned the whole story to him as if he regarded the recitation of that Sura as insufficient On that, Allah's Apostle said, "By Him in Whose Hand my soul is! That (Sura No. 112) equals one-third of the Qur'an."