It is allowed for a person to combine the Thuhr (noon) and 'Asr (afternoon) prayers, or the Maghrib (sunset) and 'Ishaa' (night) prayers, during the time of either prayer, if he is in one of the following circumstances:
1. Combining two prayers at 'Arafah and Al-Muzdalifah:
When one is performing Hajj, he should combine the Thuhr and 'Asr prayers during the time of the Thuhr prayer at 'Arafah, and the Maghrib and 'Ishaa' prayers during the time of the 'Ishaa' prayer at Muzdalifah, following the example of the Prophet .
2. Combining two prayers while travelling:
It is permissible to combine the Thuhr prayer with the 'Asr prayer and also the Maghrib prayer with the 'Ishaa' prayer during the time of either one of them while on a journey; this is regardless of whether the person is physically travelling or has made a temporary break in his journey.
Mu'ath reported that while the Prophet was in Tabook, the sun had passed its meridian and he combined the Thuhr and 'Asr prayers before starting his journey. If he started his journey before the sun passed its meridian, he would delay the Thuhr prayer until the time when he stopped for the 'Asr prayer. He would do likewise for the Maghrib prayer: If the sun set before he began his journey, he would combine the Maghrib and 'Ishaa' prayers at that time, and if he began it before the sun had set, he would combine them at the time of 'Ishaa'.[Abu Dawood and At-Tirmithi]
Kurayb, reported that Ibn 'Abbas said: "Shall I not inform you of the prayer of the Prophet during a journey?" We replied: 'Indeed!' He said: 'If the sun passed its meridian while he had stopped, he would combine the Thuhr and 'Asr prayers before remounting (i.e., moving on). If the sun had not passed its meridian while he had stopped (i.e., before breaking camp), he would travel until the time of the 'Asr prayer and would combine the Thuhr and 'Asr prayers at that time. If the sun set while he had stopped, he would combine the Maghrib and 'Ishaa' prayers. If that did not occur while he had stopped, he would ride on until 'Ishaa' time and then combine them." [Ahmad]
3. Combining two prayers during rainfall:
Al-Athram recorded in his Sunan that Abu Salamah Ibn 'Abdur-Rahman said: "It is a Sunnah to combine the Maghrib and 'Ishaa' prayers when it is raining." Al-Bukhari recorded that the Prophet combined the Maghrib and 'Ishaa' prayers on a rainy night.
4. Combining two prayers due to illness and other reasons:
Ahmad, Qaadhi Husayn, Al-Khattaabi, and Al-Mutawalli of the Shaafi'i school are of the opinion that it is allowed to combine two prayers, either during the time of the earlier or later prayer, due to illness, as this represents a greater hardship than rain. An-Nawawi, may Allah have mercy on him, said: "This is a strong opinion based on (sound) evidence." Ibn Qudaamah from the Hanbali school said in Al-Mughni it is stated: "The illness which permits one to combine the prayers is the one which would otherwise cause hardship and more weakness (if the person who is suffering from it prayed each prayer separately)."
The Hanbali school is the most accommodating in this regard as it allows one to combine the Thuhr and 'Asr prayers and the Maghrib and 'Ishaa' prayers, at the time of the early or later prayer, for one who is ill as well as for the woman who is breast-feeding and will therefore face hardship in cleaning her dress for every prayer; for the woman who is plagued by a prolonged flow of blood; a person who has urinary incontinence; the one who cannot find the means to purify himself or herself; and one who fears for his life, property, or family.
5. Combining two prayers due to some pressing need:
Imam An-Nawawi wrote in his commentary on the book of Imam Muslim : "The majority of scholars are of the opinion that it is allowed for the resident to combine prayers due to some pressing need. This is supported by the statement of Ibn 'Abbas : 'The Prophet combined his prayers because he did not want to put his Ummah (nation) to hardship, and not because of illness or any other reason."' Also, Imam Muslim stated: "The Messenger of Allah combined the Thuhr and 'Asr and then the Maghrib and 'Ishaa' prayers (all) in Madeenah, without there being any danger or rain." Ibn 'Abbas was asked: "What did he desire by that action?" He replied: "He did not want any hardship for his Ummah." Imams Al-Bukhari and Muslim recorded from him that the Prophet prayed seven Rak'ahs (units of prayer) and eight Rak'ahs, i.e., the Thuhr and 'Asr together and the Maghrib and 'Ishaa' together, in Madeenah.
Imam Muslim also recorded from 'Abdullaah Ibn Shaqeeq that 'Abdullah Ibn 'Abbas addressed the people one day after the 'Asr prayer and continued until after the sun had set and the stars began to appear (i.e., beyond the time of Maghrib). The people said to him: "The prayer! The prayer!" - and a man from the tribe of Taym in particular was continuously repeating this. Ibn 'Abbaa exclaimed: "Are you teaching me the Sunnah?" Then he said: "I saw the Messenger of Allah combining the Thuhr and 'Asr as well as the Maghrib and 'Ishaa' prayers." 'Abdullah Ibn Shaqiq commented: "I felt some uneasiness in my heart about what he had said, so I went to Abu Hurayrah to ask him about that, and he confirmed what Ibn 'Abbas had said."
6. The validity of combined prayers after their legal excuse ceases to exist:
In Al-Mughni it is stated: "If someone combines and performs both prayers at the time of the earlier prayer and then his reason for doing so ceases to exist after he has completed the prayer and before the time of the next prayer begins (the next prayer being the one that he had just prayed during the earlier time), then what he has done is sufficient for him and he needs not repeat the second prayer at its proper time. Since he performed the prayer in a proper manner, he is free from any extra obligation due to that action. He fulfilled his obligation during a circumstance in which he had a legal excuse, and his action is not invalidated by the fact that this excuse no longer exists. This is similar to the case of a person who performs Tayammum (dry ablution) and then finds water after he finishes prayer."
7. Prayer on board a ship, train or plane:
Prayer on a ship, train, plane, and so on, is valid and there is no disliking for such an act as it makes life easier for the one performing it. Ibn 'Umar said: "I asked the Prophet about prayer on ships and he replied:"Pray standing upon them unless you fear that you will drown (due to the ship capsizing).""[Ad-Daraqutni and Al-Hakim]
'Abdullaah Ibn Abi 'Utbah reported: "I accompanied Jabir Ibn 'Abdullah, Abu Sa'eed Al-Khudri and Abu Hurayrah on a boat, and they prayed standing in a congregation, with one of them as their Imam, although they could have gone ashore (if they had so desired)." [Sa'eed Ibn Mansoor] -islamweb.net