Siman 96

It is stated in Berakhot (23b): A person should not hold tefillin in his hand or a Sefer Torah in his arm and pray. This is because his mind is not settled upon him during prayer, for his heart is always upon them so that they do not fall from his hand (Rashi). And it is stated there that also a knife, and money, and a bowl, and a loaf of bread—it is forbidden to hold during the time of prayer, because he worries about them: that the knife might fall and injure him, the money might be lost, the bowl might spill, and the loaf might become soiled (ibid.). And it implies that specifically something for which there is a transgression in their falling, like a Sefer Torah and tefillin, or something where there is physical damage to his body in its falling like a knife, or monetary loss like money and a bowl and a loaf. But something for which there is no concern in their falling, like strings and handkerchiefs—it is permitted. However, there is one who says that initially it is forbidden to hold any object in his hand during the time of Shmoneh Esreh (Bach). And certainly it is so, rather the Gemara enumerated those things for which there is a complete prohibition because the distraction from them is greater. But initially one should not hold any object in his hand. And there is one who says that regarding all those which the Shas enumerated—if he held them in his hand, he must repeat the prayer (Bach). And these are wondering words, even according to the time of the Shas when they would repeat for a lack of kavanah (Taz, subsection 1 and Eliyah Rabbah). And so it is explained from the words of the Rambam, unless he said that he did not focus because of this, for then according to their time he repeats. But we do not repeat for a lack of kavanah, as I wrote in siman 93, section 2, see there.

We say in Sukkah (41a) that it is permitted to hold a lulav in one's hand during the prayer on Sukkot. And even though if it falls it might become disqualified and his heart is preoccupied with this, nevertheless, since holding it in his hand is a mitzvah in its proper time—he is not preoccupied because of it. And from this we learned that it is permitted to hold a siddur or a machzor in one's hand during Shemoneh Esrei to pray from it, for since he holds it for the sake of this prayer—it is obvious that it is no worse than a lulav. For if because taking it is a mitzvah he is not preoccupied—all the more so a mitzvah that pertains to the essence of the prayer, for one who is accustomed to pray only from the siddur. And he must hold it before Shemoneh Esrei. But during the Shemoneh Esrei—he should not seek it out while standing in Shemoneh Esrei. However, if it was prepared beforehand in the place where he prays—then it is permitted to take it even in the middle of Shemoneh Esrei. And we have already explained at the end of siman 93 that regarding the matter of praying from the siddur or praying by heart—it depends on the natures of people; see there.