All perfect praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions.
Performing Duhr and Asr prayers because of rain is one of the concessions according to our doctrine (Shafiee's doctrine) as confirmed in the Prophetic Sunna, by the Hadith Ibn Abbas (May Allah be pleased with them) who said: "The Messenger of Allah (PBUH) observed the noon and afternoon prayers together, and the sunset and Ishaa' prayers together without being in a state of fear or in a state of journey." [Moslim]. As well as, Al-Imam Malik interpreted the above narration in his book [Al-Moataa' pp.480] and stated: "I am of the view that the above concession was granted due to rain and the same was said by Al-Shafiee in [Al-Oumm vol.1 pp.94]. Besides, combining the above prayers was confirmed by two companions, Ibn Abbass and Ibn Omar (May Allah be pleased with them) and as well as the majority of Muslim scholars.
As for the other three doctrines , they didn't permit combining Duhr and Asr prayers; yet, Hanafiee's doctrine is of view that combining prayers in general isn't permissible save at the day of Arafah and Muzdalifa, while Malikee and Hanbaliee's doctrine permitted combining Maghrib (Sunset prayer) and Ishaa' (Night prayer) due to rain but not the prayers of Duhr and Asr.
It is stated in [Menah Al-Jalil vol.1 pp.420], a Malikiee's book, what follows: "It is recommended to grant a concession in combining Maghrib and Ishaa' prayers performed at the mosque if they were combined with extreme hardship but not in Duhr and Asr prayers because the degree of hardship while combining the latters is lesser than it is the case in combining the former prayers whereas the degree of hardship is greater." Further, it is stated: "Combining prayers is permissible save with Maghrib and Ishaa' prayers sol long as there was a heavy rain causing people's clothes to get very wet, in addition, a group of jurists added more conditions by which the combination the latter prayers becomes permissible if they met; if the shoes or the body got wet and there was kind of hardship while going to the Mosque." A Hanbalee's book [Kashaf Al-Qenaa' vol.2 pp.7]. Besides, Al-Bukhari narrated in his [Musnad] that the Prophet (PBUH) combined the Maghrib and Ishaa' prayers due to heavy rain as well Abu Baker, Omar and Othman (May Allah Be Pleased with all of them). It is worth noting that combining prayers is impermissible if it was cloudy or in case of light rain, that will not make peoples' clothes wet, and consequently hardship will not be the case in this situation. Nevertheless, the combination of prayers is permissible save with Maghrib and Ishaa' in case of falling snow or hail, because they have the same ruling of the heavy rain in terms of hardship."
Even so, the conditions of Shafiee's doctrine for combining Duhr and Asr prayers have to be observed as follow: combining the latter prayers due to cold or wind is impermissible even if they were strong or intense; yet, the concession for combining them is to be considered in case of falling rain, hail or snow. In this regard, it is stated: "Combining Duhr and Asr prayers is permissible as is the case with the traveler in combining prayers, even if the falling rain was light but so long as it make the clothes wet, and the same applies on melted hail and snow since they turn the wind colder and wetter similar to the case and the ruling of falling rain." [Mughni Al-Muhtajj vol.1 pp.534].
In their Fiqh books, Shafiee's jurists have mentioned the conditions for combining two prayers, and they boil down to four:
1- Making the intention to combine the two prayers during the first prayer, and at the beginning of the second.
2- Rain fell at the beginning of both prayers.
3- The rain wets the praying person`s clothes i.e. he faces hardship in going to the mosque because of it.
4- The two prayers performed in congregation at the mosque.
In addition, the permanent Imam of the mosque is the one who enjoys the authority to decide whether the conditions for combining the two prayers were met or not, and none of the prayer performers is permitted to make any objection. In principle, an Imam must have acquired a great amount of the Sharia science enabling him to settle that matter. Moreover, opening the door for debating will definitely be chaotic and over what: "rain", a blessing from Almighty Allah. However, in case one of the prayer performers had an objection, then he should discuss it with the Imam in a polite and kindness to avoid creating a scene and causing sedition at a holy place. According to scholars; enjoining kindness and forbidding inequity is permissible with conditions; some of which are: It doesn`t cause greater damage, there isn`t a considerable disagreement about the matter which is being enjoined or forbidden and inequity is expected to be removed by forbidding it.
Finally the one who follow the Imam and have an objection as regards the above matter, is obliged to talk with the Imam in this regard in private, because he isn't permitted to talk over the latter loud and in public, rather; all he can do is to inquire about the reason/reasons (evidences) of combining prayers respectfully." And Allah Knows Best.