Eruvin, Chapter 3, Mishnah 9. We continue with the question we have been examining: are the two days of Rosh Hashanah considered a single sanctity, or is the matter a doubt?
The words of the Mishnah:
Rabbi Dosa ben Hyrcanus says: "Ha'over lifnei ha'teivah b'Yom Tov shel Rosh Hashanah" - the one who leads the congregation in prayer, the chazan, on Rosh Hashanah, "omer: Hachlitzenu Hashem Elokeinu es yom Rosh HaChodesh hazeh, im hayom im l'machar" - he asks that Hashem strengthen us on this day of Rosh Chodesh, since Rosh Hashanah is also Rosh Chodesh; and in the very text of the prayer he must note that the matter is in doubt as to which is the correct day, whether today or tomorrow. "U'l'machar hu omer: im hayom im emesh" - and on the second day he says: whether today is Rosh Hashanah, or whether it was last night.
Two points in his statement:
Expressing the doubt in the text of the prayer: He relates to the matter as a doubt, and in his view the one leading the prayers must indeed say explicitly that it is a doubt.
Mentioning Rosh Chodesh: He mentions in the prayer that the day is also Rosh Chodesh.
"V'lo hodu lo Chachamim":
The Sages did not agree with him, and disputed him on both points:
Regarding mentioning Rosh Chodesh: Saying "Yom HaZikaron" fits both matters at once. It suits Rosh Hashanah, which is the day of remembrance before Hashem; and it suits Rosh Chodesh as well, concerning which it says "V'hayu lachem l'zikaron" - the blowing of the trumpets on certain days shall be a remembrance before Hashem, and among those days are counted "Roshei chodsheichem", which are likewise a time of remembrance. Therefore there is no need to mention Rosh Chodesh explicitly.
Regarding making the condition: One should not make this condition, whether today or tomorrow, because in their view the two days have a single sanctity - it is all one long day. Furthermore, there is a concern that publicizing the idea that the matter is a doubt could cause the unlearned not to treat both days seriously.
In summary: In this Mishnah, Rabbi Dosa ben Hyrcanus holds that in the Rosh Hashanah prayer one should also mention Rosh Chodesh, and should use the conditional wording "whether today or tomorrow." The Sages disputed him on both points: saying "Yom HaZikaron" already includes both matters, and one should not make a condition, both because the two days have a single sanctity and out of concern that publicizing the doubt would lead to a weakening in the honor given to both days.