Combining two prayers:
It is allowed for a person to combine the zuhr and 'asr salah, either during the time of the earlier or the later salah, or the maghrib and 'isha prayers if he is in one of the following circumstances:

Combining the salah at 'Arafa and al-Muzdalifah:
The scholars are in agreement that one is to combine the zuhr and 'asr prayer during the time of the zuhr prayer, at 'Arafa [during the performance of the pilgrimage], and the maghrib and 'isha prayers during the time of the 'isha at muzdalifah, following the example of the Prophet.

Combining the salah during traveling:
Most of the people of knowledge are of the opinion that it is permissible to combine two prayers during the time of either one of them while traveling, regardless of whether the person is actually on the road or has stopped at a place for some time.

Mu'adh reports that while the Prophet was at Tabuk and the sun had passed the meridian, the Prophet sallallahu alehi wasallam combined the zuhr and 'asr prayers before he started his journey. If he started his journey before the sun passed its meridian, he would delay the zuhr prayer until the time when he stoppped 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 'isha prayers [at that time]. If he began a journey before the sun had set, he would then combine them at the time of 'isha. This is related by Abu Dawud and at-Tirmidhi who call it hasan.

Kuraib reported that Ibn 'Abbas said: "Shall I not inform you of the salah of the Prophet sallallahu alehi wasallam during a journey?" We said: "Certainly." He said: "If the sun passed its meridian while he stopped, he would combine the zuhr 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 then he would combine the zuhr and 'asr prayers. If the sun set while he had stopped, he would combine the magrib and 'isha prayers. If that did not occur while he had stopped, he would ride until the 'isha time and then combine them." This is related by Ahmad.

Ash-Shaf'i has something similar in his Musnad, namely that when he [the Prophet] set out to travel before the sun passed its meridian, he delayed the zuhr prayer and combine it with the 'asr during the time of the 'asr salah. Al-Baihaqi recorded it with a good chain and he says: "To combine the two prayers due to traveling is something that is well-known and was practiced by the companions of the Prophet sallallahu alehi wasallam and those who followed them."

Imam Malik records in al-Muwatta' from Mu'adh that the Prophet sallallahu alehi wasallam delayed his salah one day during the battle of Tabuk and then went and prayed the zuhr and 'asr prayers together. Then he returned and went back again and said the maghrib and 'isha prayers together.

Commenting on this report, ash-Shaf'i says: "His statement, 'then he returned and left again,' only refers to a situation where the Prophet was staying in a certain place [i.e., he was not traveling from one site to another] . "

Ibn Qudamah mentions the preceding hadith and writes in al-Mughni: "Ibn 'Abdul-Barr said: 'That hadith is sahih and its chain is confirmed. The people who are familiar with the life history of the Prophet say that the battle of Tabuk took place in the ninth year of the hijrah. This hadith is a clear proof and the strongest evidence against those who claim that one can only combine the prayers while one is actually moving from one site to another as the Prophet was settled and was not traveling since the Prophet was staying in his tent and would come out and combine two prayers and then return to his tent. Muslim recorded this hadith in his Sahih and stated: 'He would pray the zuhr and 'asr together and the maghrib and 'isha together. One must follow this hadith as it is confirmed [to be authentic] and it is a clear statement on this rule and there is nothing which contradicts it. The permission to combine the salah is a concession for anyone who is traveling but it is by no means confined to just those times when the person is actually on the road [i.e., traveling from one place to another]. The same is the case for shortening the salah and for wiping over the socks, but it is best to delay it.'"

Having the intention to combine is not a condition for combining or shortening the salah. Ibn Taimiyyah holds: "That is the position of the majority of the scholars. When the Prophet combined the salah with his companions or shortened the salah with them, he never ordered any of them to make the intention for combining or shortening the salah. In fact, when he left Medinah for Makkah, he prayed two rak'at without combining the salah, and then he prayed the zuhr prayer at 'Arafa without telling the people that he intended to pray the 'asr right afterward, next he prayed the 'asr with them and they did not have the intention to combine their prayers, and in that combination he prayed the latter salah early. When he went from Medinah, he led the people in the 'asr salah at Dhul-Halifah and he did not order them to make the intention to shorten the salah."

Concerning offering the two combined prayers right after each other, Ibn Taimiyyah writes: "The correct opinion is that it is not a necessary condition to do so under any circumstances, neither during the time of the first salah nor during the time of the latter salah. There is no such limit in the shari'ah and doing so would defeat the purpose of the concession [i.e., permission to combine the two salah]." Ash-Shaf'i says: "It is quite permissible for a person to pray the maghrib in his house with the intention of combining the prayers and then go to the mosque to pray the 'isha." A similar statement has been related from Ahmad.

Combining two prayers during rain:
Al-Athram records in his Sunnan that Abu Salamah ibn 'Abdurrahman said: "It is a sunnah to combine the maghrib and 'isha prayers when it is raining." Al-Bukhari records that the Prophet sallallahu alehi wasallam combined the maghrib and 'isha prayers on a rainy night.

A summary of the opinions of the different schools of fiqh on this point follows:

The Shaf'i school says that it is allowed for the resident to combine the zuhr and 'asr or the maghrib and 'isha, praying each pair in the time of the earlier salah only, given that it is raining when one begins the earlier prayer and it is still raining by the time one begins the second prayer.

According to the Maliki school, it is allowed to combine the maghrib and 'isha in the mosque at the time of the maghrib due to rain or expected rain, if there is mud and darkness along the way, or if there is a lot of mud and it prevents the people from wearing their shoes. Nevertheless, he dislikes that the zuhr and 'asr should be combined due to rain.

According to the Hanbali school, it is only allowed to combine the maghrib and 'isha in the time of the former or the latter due to snow, ice, mud, severe cold, or rain which soaks the clothes. This concession is allowed only for one who prays with a congregation in the mosque and who comes from a distance over which he could be harmed by the rain. However, for one who prays in a congregation in his house or whose path to the mosque is covered or protected, or for one whose house is right next to the mosque, it is not allowed to combine the salah.

Combining the two prayers due to some illness or other excuse:
Ahmad, Qadi Hussain, al-Khattabi, and al-Mutawali of the Shaf'i school are of the opinion that it is allowed to combine two prayers, either during the time of the earlier or later salah, due to illness as it is a greater hardship than rain. An-Nawawi says: "This is a strong opinion based on [sound] evidence." 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 he prayed each salah separately]."

The Hanbali school is the most accommodating as it allows one to combine the prayers, at the time of the early or later salah, for one who is ill as well as for the woman who is breast-feeding and will face hardship in cleaning her dress for every salah, for the woman who is plagued by a prolonged flow of blood, for the person who cannot control his urine, and for one who cannot purify himself or herself, and for the one who fears for his life, property, or family.

Ibn Taimiyyah says: "Among the opinions the most accommodating on this question is that of the Hanbali school which allows one to combine the prayers if he is busy (since an-Nasa'i has related something to that effect from the Prophet sallallahu alehi wasallam) and they also say that the cook and baker, and so forth., may also combine their prayers if they fear their wealth (i.e., their investment or what they are working on) will otherwise be ruined."

Combining two prayers due to some pressing need:
Imam an-Nawawi writes in his commentary on Sahih Muslim: "The majority of the scholars are of the opinion that it is allowed for the resident to combine the prayers due to some pressing need. This is the statement of Ibn Sireen and Ashhab from the companions of Malik, and al-Khattabi records it from al-Qifal and ash-Shaf'i and from Abu Ishaq al-Maruzi, and from a number of as-hab al-ahadith, and it is the conclusion of Ibn al-Mundhir. This is supported by the statement of ibn 'Abbas: 'The Prophet combined his salah because he did not want to put his ummah to hardship, and not because of illness or any other reason."' The hadith from Ibn 'Abbas, mentioned previously, has been recorded by Imam Muslim who states: "The Messenger of Allah combined the zuhr and 'asr and then the maghrib and 'isha in Medinah 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." Al-Bukhari and Muslim record from him that the Prophet prayed seven rak'at and eight rak'at, i.e., the zuhr and 'asr together and the maghrib and 'isha together, in Medinah. Muslim also records from 'Abdullah ibn Shaqiq that 'Abdullah ibn 'Abbas addressed the people one day after the 'asr salah until well after the sun had set and the stars began to appear. The people said to him: "The prayer, the prayer." A man from the tribe of Taim continuously repeated: "The prayer, the prayer." Ibn 'Abbas said: "Are you teaching me the sunnah? May you have no mother." Then he said: "I saw the Messenger of Allah combine the zuhr and 'asr and the maghrib and 'isha." 'Abdullah ibn Shaqiq commented: "I felt some uneasiness in my heart about what he had said, so I went to Abu Hurairah to ask him about that, and he confirmed what Ibn 'Abbas had said."

Validity of combined prayers after their legal excuse ceases to exist:
In al-Mughni it is stated: "If someone performs both prayers at the time of the earlier salah and then his reason for doing so ceases to exist after he has completed the salah and before the time of the next salah begins [i.e., the next salah being the one which he had just prayed during the earlier time], then what he has done is sufficient for him and he need not repeat the second salah at its proper time. Since he performed the salah 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 some 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 performe tayammum, and after he finishes his salah, he finds water."