Musannaf Ibn Abi Shayba 22:85

Waki’ narrated to us, from Ibn Abi Rawwad, from al-Dahhak, who said: No man learns the Quran then forgets it except because of a sin. Then al-Dahhak recited: "And whatever of misfortune befalls you, it is because of what your hands have earned." Then al-Dahhak said: And what misfortune is greater than forgetting the Quran?