Sahih Muslim 812 a
It is reported on the authority of Abu Huraira that the Messenger of Allah (ﷺ) said:
Get together. for I am going to recite one-third of the Qur'an before you. And those who could get together gathered there. Then the Messenger of Allah (ﷺ) came out and recited:" Say: He, Allah, is One." He then entered (his house). Some of us said to the others: Perhaps there has been some news from the heaven on account of which he has gone Inside (the house). The Apostle of Allah (ﷺ) again came out and said: I told you that I was going to recite one-third of the Qur'in; keep in mind, this (Surah Ikhlas) is equivalent to one-third of the Qur'an.
وَحَدَّثَنِي مُحَمَّدُ بْنُ حَاتِمٍ، وَيَعْقُوبُ بْنُ إِبْرَاهِيمَ، جَمِيعًا عَنْ يَحْيَى، - قَالَ ابْنُ حَاتِمٍ حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، - حَدَّثَنَا يَزِيدُ بْنُ كَيْسَانَ، حَدَّثَنَا أَبُو حَازِمٍ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ احْشِدُوا فَإِنِّي سَأَقْرَأُ عَلَيْكُمْ ثُلُثَ الْقُرْآنِ ‏"‏ ‏.‏ فَحَشَدَ مَنْ حَشَدَ ثُمَّ خَرَجَ نَبِيُّ اللَّهِ صلى الله عليه وسلم فَقَرَأَ ‏{‏ قُلْ هُوَ اللَّهُ أَحَدٌ‏}‏ ثُمَّ دَخَلَ فَقَالَ بَعْضُنَا لِبَعْضٍ إِنِّي أُرَى هَذَا خَبَرٌ جَاءَهُ مِنَ السَّمَاءِ فَذَاكَ الَّذِي أَدْخَلَهُ ‏.‏ ثُمَّ خَرَجَ نَبِيُّ اللَّهِ صلى الله عليه وسلم فَقَالَ ‏"‏ إِنِّي قُلْتُ لَكُمْ سَأَقْرَأُ عَلَيْكُمْ ثُلُثَ الْقُرْآنِ أَلاَ إِنَّهَا تَعْدِلُ ثُلُثَ الْقُرْآنِ ‏"‏ ‏.‏