Deficiency of Hadiths on Forbidding Oblivion after Memorizing the Verses of the Quran

Islam encourages people to memorize the verses of holy Quran and not to forget it. Here is a question Have we committed a sin if we forget what we have memorized? Is it unlawful or not? Since there are hadiths and different views and also religious decisions which regard it as unlawful or not recommended on one hand and lack of research on the other hand make it necessary to be researched. The author of the present paper has thematically divided the issue into two categories: A) views and opinions B) related hadith in this regard.