Riyad as-Salihin, Book 9, Hadith 1011 (In-Book) ⓘ

Abu Sa'id Al-Khudri (May Allah be pleased with him) reported: A man heard another reciting Surat Al-Ikhlas repeatedly. The next morning he came to the Messenger of Allah and informed him about it as if he considered it to be of little reward. On that the Messenger of Allah said, "By Him in Whose Hand my soul is, this Surah is equal to one-third of the Qur'an."[Al- Bukhari].

وعنه أن رجل سمع رجلاً يقرأ‏:‏‏{‏قل هو الله احد‏}‏ يرددها فلما أصبح جاء إلى رسول الله صلى الله عليه وسلم، فذكر ذلك له وكان الرجل يتقالها فقال رسول الله صلى الله عليه وسلم ‏:‏‏ "‏ والذي نفسي بيده، إنها لتعدل ثلث القرآن‏" ‏‏.‏ ‏(‏‏(‏رواه البخاري ‏)‏‏)‏‏.‏