He forgot a verse whilst he was offering a naafil prayer, so he moved to pick up a Mushaf and opened it to remind himself of the verse

I was praying the two rakah sunnah salah afterMaghrib. During the
firstra'kah I was reciting a surah I just recently learned. Halfway
through I forget a verse. I stayed there for about 2 minutes
attempting to remember what the nextverse was. When I couldn't
remember, I moved forward to get a mushaf from the shelf in front of
me. I read from the mushaf and then putit back. Is what I did
permissible or is something to be avoided?.
Praise be to Allaah.
Moving during the prayer is makrooh unlessit is done out of necessity.
We have previously discussed the kinds of movements during the prayer
and the rulings ofthem in the answer the question no. 12683 .
The movement that you described is a small movement and was done in
the interests of the prayer, so there is nothing wrong with it. Itwas
one of the movements that are permitted whilst praying.
Shaykh Ibn 'Uthaymeen (may Allah have mercy on him) was asked:
If a person is offering a naafil prayer and he is reciting lengthy
soorahs and forgets some of the verses, and there is a Mushaf in front
of him, can he pick up the Mushaf in order to checkwhat he missed in
his recitation, or should he stop where he cannot carry on reciting,
and bow?
He replied:
There is nothing wrong with either option, so long as he is offering a
naafil prayer on his own.If he gets confused about a verse and the
Mushaf is nearby, there is nothing wrong with him picking it up and
looking, because this is for a need that has to dowith the prayer. And
there is nothing wrong with stopping his recitation and bowing,
because sometimes if he bows he will remember the verses that he
forgot. So there is nothing wrong with bowing then remembering them,
because this is what he wanted. And if he cannot remember them, then
he should recite whatever he can manage. End quote.
Liqa' al-Baab al-Maftooh,83/18
It should be noted that the Sunnah in the regular Sunnah prayer of
Maghrib is to recite Qul ya ayyuha'l-kaafiroon in the first rak'ah and
Qul Huwa Allahu Ahad in the second, as is proven from the Messenger
(blessings and peace of Allah be upon him). Narrated by al-Tirmidhi
(431); classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.
And Allah knows best.