Gemara

Mishnah: For any calamity, we are Masri'a (blow the Shofar) that it should not befall the Tzibur, except for too much rain.

A case occurred, and people asked Choni ha'Ma'agil to pray for rain. He told them to take their ovens inside, lest they get ruined in the rain that will come. At first he was not answered. He gave Hash-m an ultimatum, and drops of rain descended. He complained, and Hash-m sent raging rain. He complained, and Hash-m sent rain nicely. The water rose high, and people in Yerushalayim ascended to Har ha'Bayis for refuge. They asked him to pray for the water to go away. He asked them if Even To'im (a certain great stone) had been washed away.

22b - Question: Why aren't we Masri'a that there not be too much rain?

Answer (R. Yochanan): We do not pray for too much good (to go away). We learn from "...va'Harikosi Lachem Berachah Ad Bli Dai"

Question: What does it mean "Ad Bli Dai"?

Answer (Rami bar Rav): This is until your lips will wear out from saying 'Enough'!

Rami bar Rav: In Chutz la'Aretz (Bavel) we pray for this (that there not be too much rain).

Support (Beraisa): In a year with much rain, people of the Mishmar tell people of the Ma'amad to consider their brothers in Bavel, lest their houses be their graves (if they will be submerged in water; Bavel is very low).

Question: How much rain must fall before we pray that it not fall?

Answer (R. Eliezer): It must be so much that a man standing on Keren Ofel (a particular high ridge) can wet his feet. (This would never happen.)

Contradiction (Beraisa): It must be so much that a man standing on Keren Ofel can wet his hands.

Answer: These are the same (if he can wet his feet, he can bend and wet his hands).

Beraisa: "V'Nosati Gishmeichem b'Itam" - it will not be too wet or too dry. When there is too much rain it smudges the land, and produce does not grow.

23a - Beraisa: Choni's Talmidim requested 'just like you prayed for the rain to come, pray for it to go away.'

Choni: I have a tradition that we do not pray for too much good (to go away). Nevertheless, bring me a bull for Hoda'ah (thanks or confession, and to do Semichah).

Choni: Master of the world! Your children whom You brought out of Mitzrayim cannot endure too much good or too much punishment. They cannot stand up to Your anger, nor when You bestow good upon them. May it be Your will that the rain stop and relief come to the world.

Immediately, a wind came and dispersed the clouds. The sun shone, people went to the fields and found mushrooms (that resulted from the water).

Rishonim

Rif and Rosh (3:28): We are not Masri'a that there not be too much rain because we do not pray for too much good (to go away) - "...va'Harikosi Lachem Berachah Ad Bli Dai".

Ran (DH Masnisin and DH Gemara): Choni asked them if Even To'im had been washed away to hint that just like that is impossible, it is impossible to pray for excess rain to stop. Since it is Hash-m's Midah to give so much, we do not pray that He not.

Rif and Rosh: In Bavel we pray for this immediately. A Beraisa says that in a year with much rain, Anshei Mishmar tell Anshei Ma'amad to consider their brothers in Bavel, lest their houses be their graves.

Rambam (Hilchos Ta'aniyos 2:15): If there is so much rain that it is an affliction, we pray that it go away. There is no greater affliction than houses collapsing and becoming people's graves. We do not pray for rain to stop in Eretz Yisrael, for it is a land of hills and the houses are of stone and rain is helpful. We do not fast to remove what is good.

Lechem Mishneh: The Gemara also says that too much rain smudges the land, and produce does not grow. Surely this is reason to pray. The Rambam gives the greater concern.

Gemara

Shulchan Aruch (OC 576:2,11): If there is so much rain that it is an affliction, we pray that it go away. There is no greater affliction than houses collapsing and becoming people's graves. We do not pray for rain to stop in Eretz Yisrael, for it is a land of hills and the houses are of stone and rain is helpful. We do not fast to remove what is good. Nowadays in Tzefas houses often collapse from rain, so we pray that there not be excess rain.

Kaf ha'Chayim (56): The Gemara connotes that even though we do not pray for too much good (to go away), we can request, amidst Hoda'ah, that the rain cease and relief come to the world.

Shulchan Aruch (577:1): In every place, if too much rain smudges the land and prevents plowing, we are Masri'a and request mercy... We are Masri'a verbally every day, in every Tefilah. In Aneinu we say 'may it be Your will that your mercy overcome Your anger...Yisrael Your nation cannot receive too much bad or too much good. If You bestow much good upon them they cannot receive it; if You are angry at them they cannot receive it. May it be Your will that relief come to the world, and remember your mercy.'

Mishnah Berurah (1): 'In every place' includes a hilly region, where much rain is normally good.

Magen Avraham (1): The same applies if excess rain causes other problems.

Beis Yosef (DH Kosav): Part of this Tefilah is taken from R. Yishmael ben Elisha the Kohen Gadol (Berachos 7a); the rest is from Choni ha'Ma'agil.

Mishnah Berurah (6): The Darchei Moshe says that we say Aneinu after Tefilah, according to the Aleph Beis (like we do in Selichos, Aneinu Avinu Aneinu, Aneinu Bor'einu Aneinu... Our custom is that the Shali'ach Tzibur says it in Shome'a Tefilah when there is too much rain.

Kaf ha'Chayim (6): If it is urgent, we may say this in the middle of the day.