Majma al-Zawaid 975 — Forgetting knowledge

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: I think that a man forgets the knowledge he has learned because of a sin that he commits.
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its narrators have been declared trustworthy, except that al-Qasim did not hear from his grandfather.