Narrated Abu Sa`id Al-Khudri: A man heard another man reciting: Surat-ul-Ikhlas (The Unity) 'Say: He is Allah, the One (112) and he was repeating it. The next morning he came to Allah's Messenger (ﷺ) and mentioned the whole story to him as if he regarded the recitation of that Sura as insufficient On that, Allah's Messenger (ﷺ) said, "By Him in Whose Hand my soul is! That (Sura No. 112) equals one-third of the Qur'an."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، عَنْ مَالِكٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَبْدِ اللَّهِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ أَبِيهِ، عَنْ أَبِي سَعِيدٍ، أَنَّ رَجُلاً، سَمِعَ رَجُلاً، يَقْرَأُ {قُلْ هُوَ اللَّهُ أَحَدٌ} يُرَدِّدُهَا، فَلَمَّا أَصْبَحَ جَاءَ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَذَكَرَ ذَلِكَ لَهُ، وَكَأَنَّ الرَّجُلَ يَتَقَالُّهَا فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " وَالَّذِي نَفْسِي بِيَدِهِ إِنَّهَا لَتَعْدِلُ ثُلُثَ الْقُرْآنِ ".
sunnah.com