Muwatta Imam Malik Book of The Qur'an, Hadith 17

Yahya related to me from Malik from Abd ar-Rahman ibn Abdullah ibn Sasaca from his father that Abu Said al-Khudri heard a man reciting Surat al-Ikhlas (Sura 112), repeating it over and over again. In the morning he went to the Messenger of Allah ﷺ and mentioned it to him, as if he thought little of it. The Messenger of Allah ﷺ said, "Bythe One in whose hand my self is, it is equal to one third of the Qur'an."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَبْدِ اللَّهِ بْنِ أَبِي صَعْصَعَةَ، عَنْ أَبِيهِ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، أَنَّهُ سَمِعَ رَجُلاً، يَقْرَأُ {قُلْ هُوَ اللَّهُ أَحَدٌ} يُرَدِّدُهَا فَلَمَّا أَصْبَحَ غَدَا إِلَى رَسُولِ اللَّهِ ﷺ فَذَكَرَ ذَلِكَ لَهُ وَكَأَنَّ الرَّجُلَ يَتَقَالُّهَا فَقَالَ رَسُولُ اللَّهِ ﷺ " وَالَّذِي نَفْسِي بِيَدِهِ إِنَّهَا لَتَعْدِلُ ثُلُثَ الْقُرْآنِ " .