Taken from “A Summary of Islamic Jurisprudence” by Sheikh Salih Al-Fawzan , Vol 1 , pg 238 – 241
A traveler is one of the excused persons for it is permissible for him to shorten the prayer consisting of four rak’ahs ( units of prayer ) to only two rak’ahs as stated in the Noble Qur’an, the Sunnah (Prophetic Tradition ) , and the consensus of Muslim scholars. Allah , Exalted be He , says:
“And when you travel throughout the land , there is no blame upon you for the shortening prayer ..” (Qur’an : An-Nisa:101 )
Moreover , the Prophet (Peace be upon him) performed only shortened prayers on his journeys . Furthermore , on journeys , shortening the prayer is better than completing it according to the majority of scholars . It is narrated in the Two Sahihs that Aishah (may Allah be pleased with her ) said :
“When the prayer was enjoined ( by Allah ) , it was two rak’ahs (units of prayer ) only ( in every prayer ) both when in residence or on a journey . Then the prayers performed on journey remained the same , but ( the rak’ahs of ) the prayer for non-travelers were increased.” 
Umar (may Allah be pleased with him ) said:
“The prayer performed on journeys is of two rak’ahs . This is the complete prayer without shortening.” 
A Muslim on a journey is to start shortening the prayer as soon as he leaves his town or city as Allah permits the shortening of prayer for those who travel through the land . Before leaving his town, a Muslim is not legally considered a traveler through the land (and hence he is not permitted to shorten the prayer ) . The Prophet ( Peace be upon him ) used to shorten the prayer as soon as he left his hometown . So, if a person does not travel from the residential land , he is not considered a traveler.
A traveler is permitted to shorten the prayer even if he frequently travels , as in the case of a mail carrier or a taxi driver who spends most of his time on the way between towns.
It is permissible for a traveler to combine the Zuhr prayer and the Asr ( Afternoon ) prayer at the due time of either of them , and, likewise , to combine the Maghrib ( sunset ) prayer and the Isha ( night ) prayer at the due time of either of them . That is because what makes it permissible for the traveler to shorten prayer makes it permissible for him to combine prayers as well .
However , shortening prayer is a temporary permission valid when there is a necessity , as in the case when traveler is in haste on his journey. Mu’adh ( may Allah be pleased with him ) narrated :
” on the Battle of Tabuk , when the Prophet (Peace be upon him) had gone forth before the Sun passed the meridian , he would delay the Zuhar prayer and combine it with the Asr prayer , performing them together . But when he proceeded after the sun had passed the meridian , he would perform the Zuhr and the Asr prayers ( at the time of the Zuhr ) and then he would proceed. ( He acted similarly for the Maghrib prayer. ) When the sun had set before he proceeded , he would delay the Maghrib prayer and combine it with the Isha prayer , performing them together . But when he proceeded after sunset , he would perform the Isha prayer and the Maghrib prayer at the time of the Maghrib prayer “.
( Related by Ahmad and At-Tirmidhi ) 
When a traveler stops on his journey , to take a rest , it is better for him to perform each prayer shortened at its due time , not to combine prayers . If it is difficult for a Muslim patient to perform each prayer at its due time , then he is permitted to combine the Zuhr prayer and the Asr prayer ( at the duetime of either of them ), and to combine the Maghrib prayer and the Isha prayer ( at the due time of either of them ).
Shaykhul – Islam Ibn Taymiyah said :
“Allah has ordained shortening of prayer to remove difficulties and make things easy for the Muslim nation . So , a Muslim is permitted to combine prayers when necessary . All hadiths (related to this matter ) imply that it is permissible to combine two prayers at the due time of either of them in order to remove any possible hardship caused to the Prophet’s nation. Thus , it is permissible to combine prayers if it is to cause hardship if not done. This proves that it is permossible with greater reason for a Muslim patient to comine ( two ) prayers at the due time of either of them as long as it is difficult for him to perform each prayer at its due time . “ 
Ibn Taymiyah also said :
” Patients can combine the prayer according to the Sunnah for it is narrated in two hadiths that the Prophet ( Peace be upon him ) ordered a mustahadah  to combine the prayer . “
By means of analogical deduction with the case a mustahadah , the combining of prayer is permitted for every Muslim who cannot purity for every prayer, such as those inflicted with enuresis , a continuously bleeding wound , or a permanent nosebleed .The Prophet ( Peace be upon him ) said to Hamnah Bint Jahsh when she asked him about istihadah  ( vaginal bleeding other than menstruation ) :
“…But if you are strong enough to delay the Zuhr prayer and advance the Asr prayer ,to wash , and then combine the Zuhr and the Asr prayers; (and ) to delay the Maghrib prayer and advance the Isha prayer , to wash , and then combine the two prayers, do so .”
( Related by Ahmad , Abu Dawud , and At-Tirmidhi and deemed sahih ( authentic) by the latter ) 
It is permissible to combine the Maghrib prayer and the Isha prayer , in particular , if there is rain that wets clothes and causes difficulty ( for those who go to the mosque ). The Prophet ( Peace be upon him ) combined the Maghrib prayer and the Isha prayer in a rainy night  , and so did Abu Bakr and Umar afterwards .
Shaykul – Islam Ibn Taymiyah ( may Allah have mercy on him ) said :
” According to the soundest view of scholars , it is permissible for a Muslim to combine prayers because of mud or strong cold wind in a dark night , even if it has stopped raining . This is more incumbent than performing each prayer at its due time at home . Moreover, to abandon combining congregational prayers in mosque to perform prayer at home is a bid’ah ( a matter innovated in religion ) that contradicts the Sunnah ( Prophetic tradition ) . It is an act of the Sunnah to perform the Five ( obligatory ) prayers in congregation in the mosque , and this is more due , according to the unanimous agreement of Muslims , than performing prayers at home. In addition, congregational prayer combined in the mosque is more due than performing prayers at home individually, according to the unanimous agreement of the scholars who maintain the aforementioned permissibility to combine prayers ( due to bad weather ) , such as Malik , Ash-Shafi’i and Ahmad . 
It is better for one permitted to combine prayers to combine them according to what suits his / her situation most. On the day of ‘ Arafah , it is better ( for the pilgrims ) to combine the Zuhr prayer and the Asr prayer at the time of the former , but in Muzdalifah , it is better ( for the pilgrims ) to combine the Maghrib prayer and the Isha prayer at the time of the latter , according to what the Prophet (Peace be upon him) did ( in his Hajj ) . On the day of Arafah , pilgrims combine the Zuhr and the Asr prayers at the time of the former in order to continue staying at ‘ Arafah mount (without interruption), but in Muzdalifah, they combine the Maghrib and the Isha prayers at the time of the latter to continue moving toward Muzdalifah.
In general , it is an act of the Sunnah for pilgrims to combine prayers on the Day of Arafah and in Muzdalifah but in circumstances other than that , it is permissible when necessary . However , when there is no necessity , it is better for a traveller to perform each prayer at its due time . During the days of his Hajj , the Prophet (Peace be upon him) combined prayers only on the Day of ‘ Arafah and in Muzdalifah , but he (Peace be upon him) did not combine prayers in Mina because he was to stay there . So the Prophet (Peace be upon him) used to combine prayers only when he was in a hurry on a journey . We invoke Allah to guide us to useful knowledge and good deeds .
 Ad-Dara qutni ( 2275 , 2276) [2/168] and Al-Bayhaqi [3/141]
 Al-Bukhari (350) [1/601] and Muslim (1568) [3/199]
 An-Nasai (1419) [2/123] and Ibn Majah (1036) [1/556]
 Abu Dawud (1208) [2/10] and At-Tirmidhi (552) [2/438]
 See: Majmu ‘ul Fatawa (26/64)
 Mustahadah: A woman in a state of istihadah (i.e a woman having vaginal bleeding other than menstruation)
 See : Majmu ‘ul Fatawa (24/72,74)
 Istihadah: Vaginal bleeding other than menstruation
 Ahmad [6/381,382,439-440] and At-Tirmidhi (128) [1/221]
 Al-Bukhari (543) and Muslim (705)
 ‘Abdur-Razzaq in his Musannaf (4440) [2/556]
 See: Majmu ‘ul Fatawa (24/38,29)
 Muslim (2941) [4/402]
 Al-Bukhari (1674) [3/660] and Muslim (309) [5/38]