Ruling on marking the mushaf with a pencil

Question:
May Allah reward you, Honorable Shaykh. This questioner says: I attend a Qur'an memorization session, and I mark the mistakes I make with a pencil so that I do not make them again in the mushaf. Is my action permissible?

Answer:
No, in the mushaf, no, it is not permissible to write anything in it, neither with a pencil nor anything else. Keep a notebook or a piece of paper with you and write in it whatever you want, but nothing is to be put in the mushaf, nothing is to be altered in it, and no notes are to be written in the margins.