All perfect 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 established 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}.
Another evidence supporting the lawfulness of combining two prayers because of rain is 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.
Jurists have established conditions and rules for this concession; if they or one of them isn`t fulfilled, then it isn`t allowed to join prayers; consequently, each prayer must be offered on its prescribed time.
It is noticed that the excuse for joining prayers is a general one whose condition is the presence of the Illah (the effective or operative cause behind a law), which is rain that soaks people’s clothes. Therefore, the person to whom this Illah applies is allowed to join prayers.
However, the Imam is the one and only person who has the final say as regards joining prayers or not; not those praying behind him. If the stated conditions apply to even one of those following the Imam, then the latter is allowed to join the prayers. Based on this, whoever meets these conditions is allowed to join the prayers with the Imam while whoever doesn`t isn`t allowed to do so; rather, he must perform each prayer at its prescribed time.
When the Imam joins prayers because of rain, it doesn`t mean that all those praying behind him are allowed to do so. Rather, the Illah has to apply to every praying person who wants to join the 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}.
We would like to point out that if the person who meets the conditions stated before doesn`t want to join prayers, then he may do so, because this concession is neither obligatory nor recommended. Rather, it is better to leave it to avoid contradicting the scholars of the Hanafi school of thought who don`t allow joining prayers at all. Therefore, no objection should be made against those who leave the Mosala or mosque without combining prayers with the Imam.
In conclusion, it is allowed for a Muslim to combine prayers because of the rain if the conditions for doing so are met; however, this is neither obligatory nor recommended. Therefore, there is no sin on the person, who faces difficulty because of the rain, to combine prayers. Similarly, the person who leaves this concession is cleared from liability. 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.