(3) باب كَيْفَ كَانَتْ يَمِينُ النَّبِيِّ صلى الله عليه وسلم
Narrated 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 Messenger (ﷺ) and mentioned the whole story to him as if he regarded the recitation of that Sura as insufficient On that, Allah's Messenger (ﷺ) said, "By Him in Whose Hand my soul is! That (Sura No. 112) equals one-third of the Qur'an."
Reference: Sahih al-Bukhari 6643
In-book reference: Book 83, Hadith 22
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 78, Hadith 638