Siman 100

The Rambam wrote at the end of the fourth chapter: The prayers of the seasons, such as the musaf prayer of Rosh Chodesh and of the festivals – one must arrange his prayer and afterwards stand and pray, so that he does not stumble in it. Thus far his words, and it is at the end of Rosh Hashanah. And the Tur wrote that according to the opinion of the Rosh, it is not necessary on Rosh Chodesh, only on the festivals, see there. And the commentators wrote that there is no necessity (see Beit Yosef and Bach). However, from Rashi there it is implied like the Tur, see there. And our teacher, the Beit Yosef, wrote: "The prayers of the festivals and of Rosh Hashanah...". And it appears that it is a printing error, and it should say "and of Rosh Chodesh." And so it is in the Levush.

And behold, according to this, is there not also in the morning service a "prayer of intervals," namely "Ya'aleh V'yavo," and why did they write only the Musaf prayer? And it can be said: because it is a short prayer. And furthermore: because one says it many times on Rosh Chodesh in the three prayers and in Birkat HaMazon. And our teacher, the Rema, wrote that there are those who say this is specifically when praying by heart. But when praying from a siddur – it is permitted, for behold, one sees what one is praying. And such is the custom. Thus far his words. And there are those who hesitate regarding this, but it is not the primary view (see Taz).

But in truth, the custom among us is not so. Even though we pray by heart, we do not arrange [the prayer] beforehand, and even in the prayers of the Yamim Noraim, we are not accustomed to arrange [them] beforehand. And perhaps the intent of the Gemara is only regarding the shaliach tzibbur who prays in public, and not regarding the prayer of every individual. However, in the Gemara it does not imply so; see there. Several great authorities have already exerted themselves in this matter and did not produce any resolution (see Sha'arei Teshuva). And it appears to me that, behold, the Rif wrote there that even though the shaliach tzibbur only fulfills the obligation for one who is not proficient, nevertheless, when one hears from the shaliach tzibbur from beginning to end – even the proficient one has fulfilled his obligation; see there. And according to this, it can be said that this is why we pray by heart even on Rosh Hashanah and Yom Kippur, and festivals and Rosh Chodesh, for in any case: if we err in the prayer, we will hear from the shaliach tzibbur. And on Rosh Chodesh, those who travel on the way and pray in private rely on the opinion of the Tur and Rashi that we wrote in section 1. And it requires further study.