The Messenger of Allah ﷺ said, "Gather together, for I am going to recite to you one third of the Quran."So those who gathered, gathered. Then the Prophet of Allah ﷺ came out and recited, "Say, He is Allah, the One" [Ikhlas 112:1], then went back inside.Some of us said to one another, "I think news must have come to him from the heavens, and that is what made him go back inside."Then the Prophet of Allah ﷺ came out again and said, "I told you that I would recite to you one third of the Quran. Indeed, it equals one third of the Quran."
قال رسول الله ﷺ: احشدوا، فإني سأقرأ عليكم ثلث القرآن فحشد من حشد، ثم خرج نبي الله ﷺ، فقرأ: {قل هو الله أحد} ثم دخل، فقال بعضنا البعض: إني أرى هذا خبر جاءه من السماء، فذاك الذي أدخله، ثم خرج نبي الله ﷺ فقال: إني قلت لكم: سأقرأ عليكم ثلث القرآن، ألا إنها تعدل ثلث القرآن.
Muslim narrated it in the Book of Virtues of the Quran (812:261) through several routes from Yahya bin Saeed, who said, Yazid bin Kaysan told us, Abu Hazim told us, from Abu Hurairah, who said, and then he related it.His statement, "Gather," means "assemble," and the hashd is the gathering.