Musnad Ahmad Hadith 9535

Narrated from Abu Huraira (may Allah be pleased with him) that once the Noble Prophet (ﷺ) said, "Gather together, I will recite to you one third of the Qur'an." So those who were to gather, gathered. After a short while, the Noble Prophet (ﷺ) came out of the house and recited Surah Ikhlas, then went back into the house. We began saying to one another that perhaps revelation is being sent down upon the Noble Prophet (ﷺ), that is why he has gone inside. But after a short while, the Noble Prophet (ﷺ) came out and said, "I told you that I would recite to you one third of the Qur'an. Surah Ikhlas is equal to one third of the Qur'an."