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.

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 ﷺ فَذَكَرَ ذَلِكَ لَهُ وَكَانَ الرَّجُلُ يَتَقَالُّهَا فَقَالَ النَّبِيُّ ﷺ " وَالَّذِي نَفْسِي بِيَدِهِ إِنَّهَا لَتَعْدِلُ ثُلُثَ الْقُرْآنِ " .