Abu Sa'id al-Khudri said: A man heard another man reciting "Say, He is Allah, One" He was repeating it. When the next morning came, he went to the Messenger of Allah (ﷺ) and mentioned to him. The man tool it (this surah) as a small one. The Prophet (ﷺ) said: By Him in Whose Hand is my life, it is equivalent to a third of the Qur'an
SahihSahihSahihBook 8 · Hadith 46