A man heard another man reciting (Surat-Al-Ikhlas) 'Say He is Allah, (the) One.' (112. 1) repeatedly. The next morning he came to Messenger of Allah ﷺ and informed him about it as if he thought that it was not enough to recite. On that Messenger of Allah ﷺ said, "By Him in Whose Hand my life is, this Surah is equal to one-third of the Qur'an!"  

سَمِعَ رَجُلاً يَقْرَأُ {قُلْ هُوَ اللَّهُ أَحَدٌ} يُرَدِّدُهَا فَلَمَّا أَصْبَحَ جَاءَ إِلَى رَسُولِ اللَّهِ ﷺ فَذَكَرَ ذَلِكَ لَهُ وَكَأَنَّ الرَّجُلَ يَتَقَالُّهَا فَقَالَ رَسُولُ اللَّهِ ﷺ وَالَّذِي نَفْسِي بِيَدِهِ إِنَّهَا لَتَعْدِلُ ثُلُثَ الْقُرْآنِ  

My brother, Qatada bin An-Nau'man said, "A man performed the night prayer late at night in the lifetime of the Prophet ﷺ and he read: 'Say: He is Allah, (the) One,' (112.1) and read nothing besides that. The next morning a man went to the Prophet ﷺ ,~ and told him about that . (The Prophet ﷺ replied the same as (in Hadith 532) above.)  

أَنَّ رَجُلاً قَامَ فِي زَمَنِ النَّبِيِّ ﷺ يَقْرَأُ مِنَ السَّحَرِ {قُلْ هُوَ اللَّهُ أَحَدٌ} لاَ يَزِيدُ عَلَيْهَا فَلَمَّا أَصْبَحْنَا أَتَى رَجُلٌ النَّبِيَّ ﷺ نَحْوَهُ  

The Prophet ﷺ said to his companions, "Is it difficult for any of you to recite one third of the Qur'an in one night?" This suggestion was difficult for them so they said, "Who among us has the power to do so, O Messenger of Allah ﷺ?" Allah Apostle replied: " Allah (the) One, the Self-Sufficient Master Whom all creatures need.' (Surat Al-Ikhlas 112.1--to the End) is equal to one third of the Qur'an."  

عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ ؓ قَالَ قَالَ النَّبِيُّ ﷺ لأَصْحَابِهِ أَيَعْجِزُ أَحَدُكُمْ أَنْ يَقْرَأَ ثُلُثَ الْقُرْآنِ فِي لَيْلَةٍ فَشَقَّ ذَلِكَ عَلَيْهِمْ وَقَالُوا أَيُّنَا يُطِيقُ ذَلِكَ يَا رَسُولَ اللَّهِ فَقَالَ اللَّهُ الْوَاحِدُ الصَّمَدُ ثُلُثُ الْقُرْآنِ قَالَ أَبُو عَبْدِ اللَّهِ عَنْ إِبْرَاهِيمَ مُرْسَلٌ وَعَنِ الضَّحَّاكِ الْمَشْرِقِيِّ مُسْنَدٌ