Taanit 5A

רבא אמר: כיון שהתחיל, שוב אינו פוסק. וכן אמר רב ששת: כיון שהתחיל, שוב אינו פוסק.

Rava said an alternative suggestion: Once one has started to mention rain, he no longer stops, i.e., he continues the mention of rain consistently until the summer. And, so too, Rav Sheshet said: Once one has started to mention rain, he no longer stops. In other words, once one has begun to mention rain in his prayers in the additional prayer on the 8th Day of Assembly, he continues to do so uninterrupted, even in the Diaspora.

ואף רב הדר ביה. דאמר רב חננאל אמר רב: מונה עשרים ואחד יום כדרך שמונה עשרה ימים מראש השנה עד יום הכפורים, ומתחיל, וכיון שהתחיל ⁠— שוב אינו פוסק. והלכתא: כיון שהתחיל, שוב אינו פוסק.

The Talmud adds: And even Rav retracted his previously stated opinion, as Rav Ḥananel said that Rav said: One counts 21 days from Rosh HaShana, just as one counts 10 days from Rosh HaShana until Yom Kippur. And after the 21 days, one starts to mention rain, and once one has started, he no longer stops. The Talmud concludes: And the halakha is in accordance with the opinion that once one has started to mention rain, he no longer stops.

מתני׳ עד מתי שואלין את הגשמים? רבי יהודה אומר: עד שיעבור הפסח, רבי מאיר אומר: עד שיצא ניסן, שנאמר: ״ויורד לכם גשם מורה ומלקוש בראשון״.

Mishnah: Until when does one request rain? R' Yehuda says: We request rain until Passover has passed. R' Meir says: Until the month of Nisan has ended, as it is stated: “And He causes to come down for you the rain, the first rain and the last rain, in the 1st month” (Joel 2:23). Since the verse states that it rains in Nisan, the 1st month, this indicates that the entire month is considered part of the winter.

גמ׳ אמר ליה רב נחמן לרבי יצחק: יורה בניסן? יורה במרחשון הוא! דתנן: יורה במרחשון ומלקוש בניסן! אמר ליה, הכי אמר רבי יוחנן: בימי יואל בן פתואל נתקיים מקרא זה דכתיב ביה ״יתר הגזם אכל הארבה וגו׳״. אותה שנה יצא אדר ולא ירדו גשמים, ירדה להם רביעה ראשונה באחד בניסן.

Talmud: Rav Naḥman said to R' Yitzḥak: Is the first rain in Nisan? The first rain is in Marḥeshvan, as we learned in a baraita: The first rain is in Marḥeshvan and the last rain is in Nisan. R' Yitzḥak said to Rav Naḥman that R' Yoḥanan said as follows: This verse was fulfilled in the days of the prophet Joel, son of Pethuel, in a year concerning which it is written: “That which the palmer-worm has left, the locust has eaten and that which the locust has left, the canker-worm has eaten; and that which the canker-worm has left, the caterpillar has eaten” (Joel 1:4), when no crops remained. In that year, the month of Adar ended and still no rain had fallen. The rain of the first winter fell for them on the 1st of Nisan.