It was narrated from Abu Sa'eed Al Khudri that: A man heard another man reciting "Say: He is Allah, (the) One" and repeating it. When morning came he went to the Messenger of Allah (ﷺ) and told him about that. The Messenger of Allah (ﷺ) said: "By the One in Whose Hand is my soul, it is equal to one-third of the Quran."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَبْدِ اللَّهِ بْنِ عَبْدِ الرَّحْمَنِ بْنِ أَبِي صَعْصَعَةَ، عَنْ أَبِيهِ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، أَنَّ رَجُلاً، سَمِعَ رَجُلاً، يَقْرَأُ { قُلْ هُوَ اللَّهُ أَحَدٌ } يُرَدِّدُهَا فَلَمَّا أَصْبَحَ جَاءَ إِلَى النَّبِيِّ صلى الله عليه وسلم فَذَكَرَ ذَلِكَ لَهُ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " وَالَّذِي نَفْسِي بِيَدِهِ إِنَّهَا لَتَعْدِلُ ثُلُثَ الْقُرْآنِ " .
sunnah.com