The most frequent mistake made in the Amidah concerns the mention of, and the request for rain, because the wording is switched bi-annually. Throughout half the year, we become accustomed to a certain wording and tend to continue even though the time has come to change. As we learned, in three out of four possible errors regarding the request and mention of rain, we are obligated to repeat the prayer (see note 4).
If a person is uncertain as to whether or not he recited the correct words, as long as thirty days have not passed since the change in wording, in the beginning of the summer or winter, we assume that he most likely erred, since he is still in the habit of using the earlier wording. If his mistake is one of the three that necessitate a repetition, he must go back and pray correctly. However, if thirty days already passed, when people become accustomed to the change in wording; we can assume that he most likely recited the correct wording, and he does not need to repeat the Amidah.
In order to be spared this uncertainty, following which it is necessary to go back and repeat the prayer, it is best that every person accustom himself to the new wording on the day of the change by repeating it ninety times, so that his tongue will get into the habit of reciting the new wording and he will not err. In that way, even if the person is in doubt whether or not he recited the proper wording, the assumption is that he must have recited it correctly, since he already trained his tongue ninety times to say it in accordance with the halachah. Hence, it is unnecessary to repeat his prayer. (Shulchan Aruch 114:8-9).
Therefore, when the seventh night of Cheshvan arrives, according to the Sephardic minhag, which maintains that the nusach of the entire paragraph requesting rain changes, one should accustom himself to opening the berachah properly by reciting “Rofei cholei Amo Yisrael, Barech aleinu” ninety times. According to the minhag of the Ashkenazim, he says, “v’et kol minei tevuatah l’tovah, v’ten tal u’matar livrachah.” Some six months later, when he arrives at the Musaf service of the first day of Pesach, he says, “Mechayei meitim Attah rav lehoshia, Morid hatal” ninety times. On motza’ei chag before Ma’ariv of Chol HaMo’ed, according to the Sephardic minhag he says, “Rofei cholei Amo Yisrael, Barcheinu” and according to the Ashkenazic minhag, “v’et kol mini tevuatah l’tovah, v’ten berachah” (Mishnah Berurah 114:40; Kaf HaChaim 60).
. However, if he makes a mistake concerning the mention of rain in the period of transition from summer to winter, he does not need to repeat the Amidah, for as we learned, even if he did not say “Mashiv haru’ach u’morid hagashem,” but he mentioned dew, he fulfilled his obligation. Therefore, because “Morid hatal” (concerning dew) is recited in the summer, then even if he recited the summer wording, he fulfilled his obligation. (Although the Rama 114:3 writes that “Morid hatal” is not said in the summer, in Israel the Ashkenazic minhag is to say it.) However, in the transition period from winter to summer, if he erred, he invalidated his prayer, because that would mean he said “Morid hagashem” (regarding rain) in the summer. Any mistake a person makes concerning the request for rain renders his prayer invalid. To summarize, in three out of the four possibilities of error, it is necessary to repeat one’s prayer.
The source for the law that a person must repeat the Amidah in the first thirty days is in the Yerushalmi Ta’anit, chapter 1, halachah 1. Maharam of Rotenberg advises to routinize one’s tongue by saying the words ninety times. Although Rabbeinu Peretz disagrees with him, the Rosh concurs, and that is also what Shulchan Aruch rules as well. However, this is slightly problematic, for in thirty days, the second berachah of the Shemoneh Esrei is recited approximately 100 times, because of the Musaf prayers on Shabbat, holidays, and Chol HaMo’ed, whereas Birkat HaShanim is said less than eighty times, since it is not recited on Shabbat or in Musaf. Indeed, some poskim maintain that the main point is to accustom oneself with ninety Amidahs, as the Eliyah Rabbah and Derech HaChaim write. In the opinion of the Taz, Gra, and other Acharonim, the essence is to recite the prayers of those thirty days, even if he did not accustom himself to the wording ninety times. See Mishnah Berurah 114:37. In small paragraph 41 he writes in the name of the Chatam Sofer that l’chatchilah one should, indeed, accustom himself to the wording by reciting it 101 times. However, in practice, he concludes that if he only accustomed himself by reciting it ninety times, it is not in our power to rule that he should repeat the Amidah against the ruling of the Shulchan Aruch. It seems that the reasoning behind the Maharam’s words is that there is no significant difference between 80, 90, or 100 times, yet the more one repeats it, the more he will familiarize his tongue. Since the Yerushalmi established that after thirty days one most probably does not err, he established that one’s tongue can be trained by saying it ninety times, which is the average number of times the Amidah is recited in a month.