Is it permissible to hold the Mus'haf and look into it during supererogatory prayers, especially during Qiyam al-Layl, when forgetting some words in long surahs, due to the absence of someone to prompt?

It is permissible for a worshiper to recite from the Mushaf (Qur'an) by looking at it during a supererogatory prayer (nafilah), including the night vigil (qiyam al-layl). This has been authentically reported from some of the Salaf (early Muslims) and is the view of the majority of scholars.