It is reported that Al-Ḍaḥḥāk b. Muzāḥim, the famous scholar of tafsīr from the Tābiʿīn, said:

No one who has learned Qurān and then forgotten it except due to a sin he has committed, because Allāh the most high said:

وَمَآ أَصَـٰبَكُم مِّن مُّصِيبَةٍ فَبِمَا كَسَبَتْ أَيْدِيكُمْ

Qurān 42:30

“And no calamity befalls you except due to what your own hands have earnt.”

And forgetting the Qurān is one of the greatest calamities.

Al-Bayhaqī, Shuʿab Al-Īmān article 1813, and others.