The Messenger of Allah ﷺ said, "The rewards of my nation were shown to me, even down to a piece of debris that a man removes from the mosque. And the sins of my nation were shown to me, and I saw no sin greater than that of a surah or a verse from the Quran which a man was given and then forgot."
حدثنا عبد الوهاب بن الحكم الوراق البغدادي حدثنا عبد المجيد بن عبد العزيز عن ابن جريج عن المطلب بن حنطب
قال رسول الله ﷺ عرضت علي أجور أمتي حتى القذاة يخرجها الرجل من المسجد وعرضت علي ذنوب أمتي فلم أر ذنبا أعظم من سورة من القرآن أو آية أوتيها رجل ثم نسيها
Sourcing, its chain of transmission is daif. Abu Dawud narrated it in the Book of Prayer, chapter on sweeping the masjid, hadith no. 461, from Abdul-Wahhab bin Abdul-Hakam, from him. Ibn Jurayj used "an" (indicating an unverified transmission) and did not hear anything from Muttalib, and Muttalib bin Abdullah is not known to have heard from Anas, as Bukhari and others have stated. Nevertheless, Ibn Khuzayma graded it sahih, hadith no. 1297, so Hafiz said in al-Nukat 1/407, "Ibn Khuzayma overlooked its defect and included it in the chapter on masjids in his Sahih."It has a daif corroborating narration found with Abdul-Rahman bin Ahmad bin Hasan Razi in Fadail al-Quran wa Tilawatih, hadith no. 5, which contains a narrator I could not identify, along with Thawri's use of "an" (indicating an unverified transmission).