Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear brother in Islam, we would like to thank you for the great confidence you place in us, and we implore Allah Almighty to help us serve His cause and render our work for His Sake.
Generally speaking, shortening and combining the Prayers while traveling is a legal concession that portrays Islam’s tolerance and simplicity in matters of worship. The Prophet (peace and blessings be upon him) used to shorten his Prayer whenever he was on a journey. He (peace and blessings be upon him) said: “Allah likes His servants to undertake the legal concessions given to them in the same way as He likes them to observe their obligations.” (Ahmad)
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
In case you are reasonably certain that you will not be arriving at your destination before Maghrib, you must combine `Asr with Zhuhr. In other words, you must pray `Asr immediately after praying Zhuhr. This can be done with the intention of combining the two Prayers. This is a perfectly valid course for you to adopt in the particular case you have mentioned. Since it has been supported by authentic evidence in the Sunnah of the Prophet (peace and blessings be upon him), it is considered an authentic ruling in Fiqh according to the best teachings of both scholars of Fiqh and Hadith.
We read in the sources that the Prophet’s own practice was as follows: If he were to embark on his journey at the time of the first Prayer (for instance Zhuhr) and he expected to halt or dismount at the time of the second (`Asr), then he would (delay praying Zhuhr) and pray both Prayers simultaneously at the time of the second (i.e., `Asr). If, on the other hand, he were expected to journey at the time of the second (as is your case), then he would pray the second Prayer during the time of the first (i.e. pray Zhuhr first and then pray `Asr) and then proceed on his journey.
Furthermore, we read in the authentic sources that in cases such as yours where there is a real necessity for combining Prayers, it is considered permissible to do so, regardless of whether you have left the boundaries of the city or not. For according to the authentic report from Ibn `Abbas: “The Prophet (peace and blessings be upon him) prayed Zhuhr and `Asr simultaneously (one after the other), and (on another occasion) Magrhib and `Isha’ simultaneously one after the other.” The reason for doing so, according to Ibn `Abbas, was to teach the Ummah that it is permissible to do so in case of a dire necessity. The situation you mentioned is definitely a case in point.
Therefore, in cases like these, you should never hesitate to pray `Asr with Zhuhr simultaneously one after the other, for the other option, namely skipping `Asr and praying it at the time of Magrib, is simply not acceptable according to the rules of the Shari`ah.
Almighty Allah knows best.
Excerpted, with slight modifications, from: www.muslims.ca