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
“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.