אורח חיים צ״וOrach Chayim 96

Se’if 1א׳

כְּשֶׁהוּא מִתְפַּלֵּל, לֹא יֶאֱחֹז בְּיָדוֹ תְּפִלִּין וְלֹא סֵפֶר מִכִּתְבֵי הַקֹּדֶשׁ וְלֹא קְעָרָה מְלֵאָה וְלֹא סַכִּין וּמָעוֹת וְכִכָּר, מִפְּנֵי שֶׁלִּבּוֹ עֲלֵיהֶם שֶׁלֹּא יִפְּלוּ וְיִטָּרֵד וְתִתְבַּטֵּל כַּוָּנָתוֹ. וְלוּלָב בִּזְמַנּוֹ, מֻתָּר לֶאֱחֹז בַּיָּד, כֵּיוָן שֶׁהָאֲחִיזָה בְּיָדוֹ הִיא מִצְוָה אֵינוֹ נִטְרָד בִּשְׁבִילוֹ.

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.

Se’if 2ב׳

מֻתָּר לֶאֱחֹז מַחֲזוֹר תְּפִלּוֹת בְּיָדוֹ בְּשָׁעָה שֶׁמִּתְפַּלֵּל, הוֹאִיל וְתוֹפֵס לְצֹרֶךְ תְּפִלָּה עַצְמָהּ לָא טָרִיד, דּוּמְיָא דְּלוּלָב דְּמִשּׁוּם דִּלְקִיחָתוֹ מִצְוָה לָא טָרִיד. הַגָּה: וּמִכָּל מָקוֹם אִם לֹא הָיָה בְּיָדוֹ קֹדֶם שֶׁהִתְחִיל, לֹא יַחֲזֹר אַחֲרָיו בַּתְּפִלָּה לִטְּלוֹ, אֶלָּא אִם כֵּן הָיָה בְּמָקוֹם מְיֻחָד שֶׁהוּא מוּכָן, אָז מֻתָּר לִטְּלוֹ אֲפִלּוּ בְּתוֹךְ הַתְּפִלָּה, כְּדֵי לְהִתְפַּלֵּל מִתּוֹכוֹ (הר"י פֶּרֶק מִי שֶׁמֵּתוֹ).

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")