Shulchan Arukh Orach Chayim · Siman 96

English

That One Should Prevent All Distractions in Order to Concentrate, containing 2 s'ifim. When one is praying, one should not hold in one's hand tefillin, and not a book of holy writings, and not a full plate, and not a knife, or money, or a loaf [of bread] because one's heart is [focused] on them that they should not fall, and one will be distracted and will lose one's focus. And [regarding] a lulav at its time [i.e. performing its mitzvah]: it is permitted to hold it in [one's] hand, [and] since the holding [of it] in one's hand is the mitzvah, one will not be distracted because of it.

English

It is permitted to hold a prayerbook in one's hand at the time that one is praying, [and] since one is holding it for the purpose of the prayer itself, one is not distracted [by it]. It is similar to [holding] a lulav, that since its "taking" is the mitzvah, one is not distracted. Gloss: Nevertheless, if [the prayer book] was not in one's hand before one started [praying], one should not go looking for it during [the middle of the] prayer [i.e. Amidah] to pick it up, unless it is the case that it was in a particular place which one prepared. Then it is permitted to pick it up, even in the middle of the prayer, in order to pray from it (Rabbeinu Yonah - [Tractate Berachot - Chapter 3] "Mi Shemeito")