Sahih al-Bukhari 7374
صحيح البخاري#7374SahihNarrated Abu Sa`id Al-Khudri:A man heard another man reciting (in the prayers): 'Say (O Muhammad): "He is Allah, the One." (112.1) And he recited it repeatedly. When it was morning, he went to the Prophet (ﷺ) and informed him about that as if he considered that the recitation of that Sura by itself was not enough. Allah's Messenger (ﷺ) said, "By Him in Whose Hand my life is, it is equal to one-third of the Qur'an
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 97:4