What is the ruling on forgetting the Qur'an?

Forgetting the Qur’an is negligence and a sin, and many of the Salaf considered it among the major sins, citing the words of Allah Almighty: "And whatever strikes you of disaster - it is because of what your hands have earned" (Quran 42:30), and forgetting the Qur’an is one of the greatest disasters. They also cited a marfu’ hadith: "The sins of my Ummah were presented to me, and I did not see a sin greater than a surah from the Qur’an given to a man then he forgot it." Likewise, there are narrations from the Salaf who considered forgetting the Qur’an among the greatest sins. Turning away from recitation leads to forgetting the Qur’an, and forgetting it indicates a lack of care and trivialization of its matter, and returning to ignorance after knowledge is severe.