Abu Dawud Book 2, Hadith Number 1469.
Chapter : Strictness about the person who memorizes the quran and then forgets it.
Narated By Sa’d ibn Ubadah : The Prophet (PBUH) said: No man recites the Qur’an, then forgets it, but will meet Allah on the Day of Judgment in a maimed condition (or empty-handed, or with no excuse).