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 ﷺ and informed him about it as if he considered it to be of little reward. On that the Messenger of Allah ﷺ 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 ﷺ، فذكر ذلك له وكان الرجل يتقالها فقال رسول الله ﷺ :" والذي نفسي بيده، إنها لتعدل ثلث القرآن". ((رواه البخاري )).