(13) باب فَضْلِ قُلْ هُوَ اللَّهُ أَحَدٌ
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!"
Reference: Sahih al-Bukhari 5013
In-book reference: Book 66, Hadith 35
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 6, Book 61, Hadith 533