Narrated Abu Said Al-Khudri:

A man heard another man reciting (Surat-Al-Ikhlas) 'Say He is Allah, (the) One.' (112. 1) repeatedly. The next morning he came to Allah's Apostle and informed him about it as if he thought that it was not enough to recite. On that Allah's Apostle ﷺ said, "By Him in Whose Hand my life is, this Surah is equal to one-third of the Qur'an!" Narrated Abu Said Al-Khudri: My brother, Qatada bin An-Nau'man said, "A man performed the night prayer late at night in the lifetime of the Prophet 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 ,~ and told him about that . (The Prophet replied the same as (in Hadith 532) above.)

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا مَالِكٌ، 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 رَسُولِ اللَّهِ ﷺ فَذَكَرَ ذَلِكَ لَهُ وَكَأَنَّ الرَّجُلَ يَتَقَالُّهَا فَقَالَ رَسُولُ اللَّهِ ﷺ " وَالَّذِي نَفْسِي بِيَدِهِ إِنَّهَا لَتَعْدِلُ ثُلُثَ الْقُرْآنِ ".