From Sahih Bukhari (Hadith 7374):
Narrated 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
Reference: Bukhari 7374