Is it permissible for a Hafiz to open the Mushaf during prayer when forgetting a verse?

Movement during prayer is disliked unless there is a need for it. The movement of taking a mushaf (Quran) due to forgetting verses is a slight movement for the benefit of the prayer; there is no harm in it, and it is considered among the permissible movements. This is supported by the fatwa of Sheikh Ibn 'Uthaymeen. The Sunnah for the regular Maghrib prayer is to recite "Al-Kafirun" and "Al-Ikhlas."