Combining two Salah at one time

Just as it is a grave sin to make a Salah Qaza, likewise, combining two Salah at one time is a grave sin too. For example, if a person performs the Salah of Zohr and A'ssr at the time of A'ssr or at the time of Zohr, it is a very grave sin because by performing Salah in this way, the person goes against the Qur’an-Kareem and the Hadith that command to perform every Salah at its prescribed time. According to a Hadith found in Tirmizi which is reported by Hazrat Abdullah Bin Abbaas (Radiyallahu Anhu), Rasulullah (Sallallaahu Alaihi Wasallam) has said that a person who combines two Salah at one time without a valid reason, has committed one of the major sins. 

It is true that Nabi (Sallallaahu Alaihi Wasallam) had combined Zohr and A'ssr at the time of Zohr in the plain of ‘Arafah. Similarly, he had combined Maghrib and ‘Isha at the time of ‘Isha in Muzdalifah. This is an exception that is valid for the Haji to practice under certain conditions until today also. But besides that, it is a very grave sin to do so. There are Hadith that gives the impression that Nabi (Sallallaahu Alaihi Wasallam) used to perform two Salah at one time on journeys. However, according to more detailed Hadith, Nabi (Sallallaahu Alaihi Wasallam) used to perform every Salah at its time without going beyond its time. That is, he was performing the Salah of Zohr at its ending time and the Salah of A'ssr at its beginning time. Similarly, he used to perform the Salah of Maghrib at its ending time and the Salah of ‘Isha at its beginning time. Every Salah was performed at its time although both seemed to be together at the same time. According to a Hadith found in Tabrani that is reported by Hazrat Mu'az Bin Jabal (Radiyallahu Anhu), Rasulullah (Sallallaahu Alaihi Wasallam) was performing the Salah of Zohr at its ending time and the Salah of A'ssr at its beginning time in the battle of Tabuk. Similarly, he used to perform the Salah of Maghrib at its ending time and the Salah of ‘Isha at its beginning time. This way of combining Salah was also reported by Hazrat Ali (Radiyallahu Anhu), Hazrat Abdullah Bin Mas'oûde (Radiyallahu Anhu) and Hazrat Aisha (Radiyallahu Anha). Thus, it is not permissible to shift a Salah from its time. If there is a necessity, one Salah can be performed at its ending time and the other at its beginning time. Virtually, it seems that both were performed at one time but in reality each was performed at its time.

[English version of "Combine deux namaz dans ene seul l'heure" – Al Jamiat – Vol.3 No.11]