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.
In principle, each prayer must be performed at its prescribed time, because Allah the Almighty says {What means}: " For such prayers are enjoined on believers at stated times." {An-Nisa`, 103}. However, Islamic Sharia has allowed advancing or delaying prayers in some circumstances: A non-traveler is allowed to combine Duhr and Asir at the time of Duhr because of rain while it is allowed to combine Maghrib and `Isha` at the time of Maghrib for a traveler.
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.
Combining two prayers is the concession; not the principle, and concessions are made use of only upon meeting their reasons and conditions, and this is expressed by the scholars through the Sharia maxims: "Concessions are only made use of in case of certainty." If the reasons and conditions of a concession are met, then it is permissible, not obligatory, to combine two prayers. However, if they or one of them isn`t met or there is doubt about their being met, then it isn`t allowed to combine the two prayers; consequently, each prayer must be offered at its prescribed time since this is the principle: the duty imposed by Allah. In addition, it is the duty of every Muslim to offer obligatory acts of worship in their perfect form to be cleared from liability before Allah since it isn`t allowed to incline to concessions without making sure that their reasons and conditions are actually met. This is particularly true because some scholars haven`t allowed combining two prayers in the first place. Accordingly, how dare some say, without certainty, that combining two prayers is a Sunnah?!
Moreover, there are types for concessions as regards making use of them. Imam An-Nawawi {May Allah have mercy on him} said: "Concessions are divided into several types; one of which is obligatory concession and its forms…..The second type: A concession which is better be left, such as wiping over Khufs. This because our fellow scholars are agreed that it is better to wash one`s feet, and the evidence on this has been mentioned in the related section. Similarly, scholars are agreed that it is better to leave combining prayers." {Al-Majmou` Shareh Al-Mohathab, Vol.4: P.336}.
In conclusion, it is allowed to combine two prayers under the excuse of rain; provided that the stipulated conditions are met, since this is a concession. However, whoever leaves this concession has performed the duty imposed by Allah, which is the principle, and none of the scholars can object to that. This is particularly true when one is unsure of having met the conditions for the concession or have taken them lightly, and the extent of the reward will be in accordance with the extent of the trial. And Allah the Almighty knows best.