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.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا الْقَعْنَبِيُّ، عَنْ مَالِكٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَبْدِ اللَّهِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ أَبِيهِ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، أَنَّ رَجُلاً، سَمِعَ رَجُلاً، يَقْرَأُ { قُلْ هُوَ اللَّهُ أَحَدٌ } يُرَدِّدُهَا فَلَمَّا أَصْبَحَ جَاءَ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَذَكَرَ ذَلِكَ لَهُ وَكَانَ الرَّجُلُ يَتَقَالُّهَا فَقَالَ النَّبِيُّ صلى الله عليه وسلم " وَالَّذِي نَفْسِي بِيَدِهِ إِنَّهَا لَتَعْدِلُ ثُلُثَ الْقُرْآنِ " .
sunnah.com