Shulchan Arukh, Orach Chayim Siman 100
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תפלות של מועדות ושל ראש חודש צריך להסדיר תפלתו קודם שיתפלל כדי שתהא שגורה בפיו: הגה וי"א דוקא כשמתפללים על פה אבל כשמתפללין מתוך הסדור מותר דהא רואה מה שמתפלל וכן נוהגין (ב"י בשם הר"ר מנוח וכ"כ תניא בשם ר"א):
[Regarding] prayers [i.e the Amidah] of the festivals and of the new month, one needs to organize one's prayer before one prays so that it will be fluent on his tongue.Gloss: And there are those who say that [this is] specifically when we pray by heart, but when we are praying from within a prayer book, it is permitted [to pray without organizing] since one sees what one is praying, and this is what we practice. (Bet Yosef in the name of Rav R. Manoah, and so too wrote Tanya in the name of R.A.