Language:
كتاب فضائل القرآن
66  
Virtues of the Qur'an
(13)
Chapter: The superiority of Qul-Huwa Allahu Ahad (Surat Al-Ikhlas) (No.112)
(13)
باب فَضْلِ قُلْ هُوَ اللَّهُ أَحَدٌ
Sahih al-Bukhari 5013

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 Messenger (ﷺ) and informed him about it as if he thought that it was not enough to recite. On that Allah's Messenger (ﷺ) said, "By Him in Whose Hand my life is, this Surah is equal to one-third of the Qur'an!"

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا مَالِكٌ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَبْدِ اللَّهِ بْنِ عَبْدِ الرَّحْمَنِ بْنِ أَبِي صَعْصَعَةَ، عَنْ أَبِيهِ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، أَنَّ رَجُلاً، سَمِعَ رَجُلاً، يَقْرَأُ ‏{‏قُلْ هُوَ اللَّهُ أَحَدٌ‏}‏ يُرَدِّدُهَا فَلَمَّا أَصْبَحَ جَاءَ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَذَكَرَ ذَلِكَ لَهُ وَكَأَنَّ الرَّجُلَ يَتَقَالُّهَا فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ وَالَّذِي نَفْسِي بِيَدِهِ إِنَّهَا لَتَعْدِلُ ثُلُثَ الْقُرْآنِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 5013
In-book reference : Book 66, Hadith 35
USC-MSA web (English) reference : Vol. 6, Book 61, Hadith 533
  (deprecated numbering scheme)