Narrated Anas ibn Malik: The Prophet (ﷺ) said: The rewards of my people were presented before me, so much so that even the reward for removing a mote by a person from the mosque was presented to me. The sins of my people were also presented before me. I did not find a sin greater than that of a person forgetting the Qur'anic chapter or verse memorised by him.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَبْدُ الْوَهَّابِ بْنُ عَبْدِ الْحَكَمِ الْخَزَّازُ، أَخْبَرَنَا عَبْدُ الْمَجِيدِ بْنُ عَبْدِ الْعَزِيزِ بْنِ أَبِي رَوَّادٍ، عَنِ ابْنِ جُرَيْجٍ، عَنِ الْمُطَّلِبِ بْنِ عَبْدِ اللَّهِ بْنِ حَنْطَبٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " عُرِضَتْ عَلَىَّ أُجُورُ أُمَّتِي حَتَّى الْقَذَاةُ يُخْرِجُهَا الرَّجُلُ مِنَ الْمَسْجِدِ وَعُرِضَتْ عَلَىَّ ذُنُوبُ أُمَّتِي فَلَمْ أَرَ ذَنْبًا أَعْظَمَ مِنْ سُورَةٍ مِنَ الْقُرْآنِ أَوْ آيَةٍ أُوتِيَهَا رَجُلٌ ثُمَّ نَسِيَهَا " .
sunnah.com