Is it a sin for one who memorized part of the Qur'an and then forgot it?

Q: Sister M. M. 'A. from Sana'a says in her question: I memorized a complete part of the Noble Qur'an, but because there was no one to listen to me regularly, I forgot it. So, is there a sin on me? And should I memorize it again?
A: There is no sin on you, if Allah wills, in that; because of the saying of Allah the Exalted: [20:115] and the saying of the Prophet (peace and blessings be upon him): And because forgetfulness overcomes a person, and he cannot be safe from it.
As for the warning that has been reported in that regard from the Prophet (peace and blessings be upon him), it is weak. It is prescribed for you to strive to memorize what is easy of the Book of Allah, especially the Hizb al-Mufassal, so that you can thereby read in your prayer what is easy of it after al-Fatihah. As for al-Fatihah, memorizing it is obligatory because it is a pillar in the prayer in every rak'ah; because of the saying of the Prophet (peace and blessings be upon him): It is agreed upon as authentic. And by this it is known that reciting it is a pillar in the prayer in the obligatory and voluntary prayers for the imam and the one praying alone.
As for the ma'mum, it is obligatory for him according to the correct of the scholars' opinions, and it is dropped for him in the case of forgetfulness and ignorance, and in the case where he catches up with the imam while he is bowing or at the time of bowing and he is unable to recite it; because of the saying of the Prophet (peace and blessings be upon him): We said: Yes. He said: It was narrated by Imam Ahmad, Abu Dawud, al-Tirmidhi, and Ibn Hibban with a sound chain, from 'Ubadah ibn al-Samit (may Allah be pleased with him).
And since it is established in Sahih al-Bukhari from Abu Bakrah (may Allah be pleased with him) that he came to the mosque and the Prophet (peace and blessings be upon him) was bowing, so he bowed before he reached the row, then he entered the row. When the Prophet (peace and blessings be upon him) finished the prayer, it was mentioned to him, so he said to him (peace and blessings be upon him): And he did not order him to make up the rak'ah. This indicates that if the ma'mum does not catch up with the recitation with the imam because he came near the bowing, then the rak'ah suffices for him. And like that is the ma'mum who forgot it or did not know it, like the other obligatory matters in the prayer. And Allah is the One who grants success.