Anas bin Malik reported,
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."
Abu Dawud 461
Daifحدثنا عبد الوهاب بن عبد الحكم الخزاز أخبرنا عبد المجيد بن عبد العزيز بن أبي رواد عن ابن جريج عن المطلب بن عبد الله بن حنطب
عن أنس بن مالك قال
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 is daif. Tirmidhi narrated it in "The Virtues of the Quran," in the chapter, "I have not seen a sin greater than a surah given to a man who then forgot it," hadith 2916, from Abdul-Wahhab Warraq Baghdadi with this wording, and said it is "isolated." Ibn Jurayj was a mudallis, as mentioned before, hadith 19, and he did not hear anything from Muttalib, and Muttalib did not hear from Anas 
. Ibn Khuzayma graded it sahih, 1297. See also Nukat Ziraf 1/407.