Sa'ad ibne-'Ubadah Radiyallahu 'anhu narrates that Rasullullah Sallallahu 'alaihi wasallam said: There is not a person who learns to read the Qur'an and then forgets it, except that he will meet Allah on the Day of Resurrection as a leper. (Abu Dawud: 1474)
Note: Many meanings have been given to the forgetting of the Qur'an.
One of them is that, one is unable to recite despite looking at the
pages of the Qur'an. Another is that he is unable to recite it from
memory. The third is that he is negligent about reciting it. The fourth
is that after being aware of the commandments in it, he does not act
upon them. (Badhl-ul-Majhud, Sharh Sunan Abu Dawud Eini)