Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

The lawfulness of combining prayers because of rain is a concession granted by Sharia, and confirmed in the Prophetic Sunnah. Yahya related to me from Malik from Abu'z Zubayr al-Makki from Said ibn Jubayr that Abdullah ibn Abbas said, "The Messenger of Allah, may Allah bless him and grant him peace, prayed dhuhr and asr together and maghrib and isha together, and not out of fear nor because of travelling."{Related by Muslim and Malik in his book "Al-Mowatta`}. Malik then added, "This is because of rain." Imam Al-Shafi also supported this view` in his book {Al-Omm, Vol.1:P.94}.

The evidence on this is that Ibn 'Abbas reported that the Messenger of Allah (Peace and blessings be upon him) combined the noon prayer with the afternoon prayer and the sunset prayer with the 'Isha' prayer in Medina without being in a state of danger or rainfall {Transmitted by Muslim}.

This was also confirmed by the narrations reported on the authority of Ibn Abbas and Ibn Omar (May Allah be pleased with them), and this is also the view of the majority of the Maliki, Shafie, and Hanbali scholars from Ahlus Sunnah Wal Jamaah.

This concession came to facilitate for the Muslim Ummah and spare it unnecessary hardship. "I said to Ibn 'Abbas: What prompted him to do that? He said: So that his (Prophet's) Ummah should not be put to (unnecessary) hardship."{Transmitted by Muslim}. In their books, scholars have mentioned in detail the conditions for joining prayers, and they (Conditions) were tied with the presence of hardship since joining prayers was intended to remove hardship, which is evident in the words of the scholars. On their part, Shafie scholars stipulated that the rain soak the clothes of the praying person and considered this to be the Illah (the effective or operative cause behind a law) of the Hardship. Based on this,and according to Shafie scholars, snow that doesn`t soak the clothes doesn`t permit joining prayers. Sheikh Al-Islam Zakaria Al-Ansari (May Allah have mercy on him) said: "It isn`t permissible to join prayers because of snow and hail that melt away." {Asna Al-Matalib, Vol.1:P.245}.

Amongst the conditions set by the scholars for the permissibility of joining prayers because of rain is facing hardship while coming to the mosque, and based on this, the person living next to the mosque isn`t allowed to join prayers. Al-Shirbini (May Allah have mercy on him) said: "According to the more correct opinion mentioned by An-Nawawi in his book (Al-Rawdah), this concession pertains to the person praying in congregation in a Mosala (Prayer place), inside or outside the mosque. And-according to custom- this Mosala has to be far from the door of his house to the extent that he faces difficulty, because of the rain, while going there. However, this concession doesn`t apply to the person praying at home alone or in congregation, the person who walks to the prayer place under a shelter, or the person who lives close to the Mosala. These persons aren`t allowed to join prayers because they don`t face difficulty because of the rain." {Moghni Al-Mohtaj, Vol.1:P.534}. The Maliki scholar Al-Mawoaaq (May Allah have mercy on him) said: "It isn`t permissible to join Duhr and Asir because people are busy with their businesses and daily activities while it is permissible to join Maghrib and `Isha` because it is dark." {At-Taj waliklill, Vol.2:P.514}. Imam Malik (May Allah have mercy on him) said: "The Prophet (PBUH) joined Maghrib and Isha` because of the rain out of mercy to the people." {Al-Modawanah, Vol.1:P.204}.

In conclusion, if Sunnah means whatever the Prophet (PBUH) said, did, or approved of, then joining prayers in this sense is from the Sunnah because it was confirmed that he (PBUH) did combine prayers because of the rain. However, he did so to show the people that this is permissible, and He (PBUH) may do what is disliked to show permissibility. 

However, if the Sunnah means that the person who does it is rewarded, then combining prayers in this sense isn`t considered from the Sunnah, so the one who combines prayers doesn`t receive an extra reward over the one who doesn`t. On the contrary, the one who offers each prayer at its prescribed time does the duty originally imposed by Allah, in addition to the reward for going to the mosque and waiting for each prayer at its due time.

In conclusion, joining prayers because of rain after meeting the necessary conditions was legislated to remove hardship, so it is a type of concession and a form of facilitation whose lawfulness was confirmed in the Sunnah and not as a Sunnah for which a person is rewarded; particularly since some scholars don`t allow joining prayers in the first place. Imam An-Nawawi {May Allah have mercy on him} said: "……….Scholars are agreed that it is better to leave combining prayers." {Al-Majmou` Shareh Al-Mohath-thab, Vol.4: P.336}. And Allah the Almighty knows best.