The Prophet (saw) is reported to have said, ‘The sins of my nation were presented before me, and I saw no sin to be greater than that of a man who memorized a verse and then forgot it.‘ Is this hadeeth authentic?
I do not know of this hadeth; I have not read of it anywhere. Nonetheless, there are two kinds of forgetting:
When one forgets because one is distracted or becomes sick: one who forgets for these reasons is not held accountable.
When one forgets because one turns away from the Qur’aan and its recitation: such a person will be held accountable because his forgetfulness was a result of his (own) negligence.
[Taken from "Islamic Rulings Regarding The Qur'aan", Pp. 22-23. Quoting from "Fataawa Al-Fawzaan"]