Narrated Al-Muttalib bin 'Abullah bin Hantab:
from Anas bin Malik that the Messenger of Allah (ﷺ) said: "The rewards for my Ummah were displayed before me, even (the reward for) the dust that a man comes out of the Masjid with. The sins of my Ummah were displayed before me, and I have not seen a sin worse than a Surah or Ayah of the Qur'an which a man learned and then forgot." (DE)
English reference (DE): Vol. 5, Book 42, Hadith 2916 (DE)
Arabic reference (DE): Book 45, Hadith 3166 (DE)