The Messenger of Allah ﷺ said, "Gather together, for I will recite to you a third of the Quran." So those who gathered, gathered. Then he came out and recited, "Say, He is Allah, the One." Then he went back inside.Some of us said to one another, "This must be news that came to him from the heavens, and that is what made him go back in."Then he came out and said, "I told you that I would recite to you a third of the Quran, and indeed it equals a third of the Quran."
حدثنا يحيى بن سعيد قال حدثنا يزيد بن كيسان قال حدثني أبو حازم
قال رسول الله ﷺ احشدوا فإني سأقرأ عليكم ثلث القرآن قال فحشد من حشد ثم خرج فقرأ قل هو الله أحد ثم دخل فقال بعضنا لبعض هذا خبر جاءه من السماء فذلك الذي أدخله ثم خرج فقال إني قد قلت لكم إني سأقرأ عليكم ثلث القرآن وإنها تعدل ثلث القرآن
Its chain of transmission is sahih. Bukhari narrated it, 4/1916, no. 4727, in the chapter on the virtues of the Quran, the virtue of "Say, He is Allah, the One", with similar wording. Muslim also narrated it, in the book of the prayer of travelers, the virtue of reciting "Say, He is Allah, the One", 1/557, no. 812. And Tirmidhi narrated it, 5/168, no. 2900, in the virtues of the Quran, what has been said about the surah.