A man heard another man reciting "Say, He is Allah, the One" [Ikhlas 112:1], repeating it over and over. When morning came, he went to the Messenger of Allah ﷺ and mentioned it to him, as though the man considered the surah to be insignificant.The Messenger of Allah ﷺ said, "By the One in whose hand my soul rests, it equals a third of the Quran."
أن رجلا سمع رجلا يقرأ: {قل هو الله أحد} يرددها، فلما أصبح جاء إلى رسول الله ﷺ، فذكر ذلك له وكأن الرجل يتقالها- فقال رسول الله ﷺ: والذي نفسي بيده، إنها لتعدل ثلث القرآن.
Bukhari related it in the Excellences of the Quran (5013) from Abdullah bin Yusuf, who said, Malik informed us, from Abdul-Rahman bin Abdullah bin Abdul-Rahman bin Abu Saasaa, from his father, from Abu Saeed Khudri, who narrated it.