The Messenger of Allah ﷺ said, "Gather together, for I shall recite to you a third of the Quran." So those who could gather did so. Then the Prophet of Allah ﷺ came out and recited, "Say, He is Allah, the One," then went back inside.We said to one another, "The Messenger of Allah ﷺ said, 'I shall recite to you a third of the Quran.' I think this must be news that has come from the heavens."Then the Prophet of Allah ﷺ came out again and said, "I told you that I would recite to you a third of the Quran. Indeed, this surah is equal to a third of the Quran."
حدثنا محمد بن بشار حدثنا يحيى بن سعيد حدثنا يزيد بن كيسان حدثنا أبو حازم
قال رسول الله ﷺ احشدوا فإني سأقرأ عليكم ثلث القرآن قال فحشد من حشد ثم خرج نبي الله ﷺ فقرأ قل هو الله أحد ثم دخل فقال بعضنا لبعض قال رسول الله ﷺ فإني سأقرأ عليكم ثلث القرآن إني لأرى هذا خبرا جاء من السماء ثم خرج نبي الله ﷺ فقال إني قلت سأقرأ عليكم ثلث القرآن ألا وإنها تعدل ثلث القرآن
Muslim also narrated it in "Prayer of the Traveler," chapter on the merit of reciting "Say, He is Allah, He", hadith no. 812, from the hadith of Yahya Qattan through the same chain.