8. Prayer (Kitab Al-Salat) Detailed Injunctions about Witr - كتاب الوتر
21) Chapter: Severe Reprimand For Whoever Memorized The Qur'an And Then Forgot It
Narrated Sa'd ibn Ubadah:
The Prophet (ﷺ) 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).