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].
App reference: Book 9, Hadith 21
Arabic/English book reference: Book 9, Hadith 1011