Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.088.

Zhakwan, the protege of ‘Aishah, would lead her in prayer during Ramadan while reciting from a copy of the Qur’an. This is related by Malik. Ash-Shaf’i’s opinion is that it is allowable. An-Nawawi holds: “If one sometimes turns pages during a salah, it does not invalidate it. If he looks at something that is written which is not the Qur’an and he reads it to himself, it does not invalidate the salah, even if it is done for a long period of time, nevertheless, it is a hated act.” Ash-Shaf’i has made a statement about it in al-Imla’.