Indeed, Prayer occupies a great status in Islam that is not shared by any other act of worship. It is a pillar for which region cannot stand without it. According to a hadith narrated by Mu’adh ibn Jabal (may Allah be pleased with him), the Messenger of Allah (peace be upon him) said: “Shall I not tell you of the head of the whole matter and its pillar and top?” I (Mu`adh) said, “Yes, O Messenger of Allah.” He said, “The head of the matter is Islam, its pillar is prayer and its top is jihad.” (Narrated by Al-Tirmidhi
Also, Salah is the second in status to the two testimonies of faith. The Messenger of Allah (peace be upon him) said: “Islam is built on five [pillars]: the testimony that there is no god but Allah and that Muhammad is His slave and Messenger, establishing prayer, paying zakat, performing pilgrimage to the House, and fasting Ramadan.” (Narrated by Al-Bukhari)
On the question of shortening prayer while on journey, Dr. Rajab Abu Mleeh said that one should combine both Zuhr and `Asr Prayers before going out on a trip, offering them in complete Rak`ahs (i.e. not shortened, each prayer in four Raka`ahs), according to the preponderant opinion of scholars. They maintain that shortening prayer is only allowed when the traveler goes past the boundaries of his district.
Then, if this is not possible, it becomes obligatory on a traveler to offer Salah when the bus stops whether in a mosque or by the side of the road, and in this case, it is desirable to shorten the `Asr Prayer. He may ask the bus chauffer to stop so that he performs prayer on its due time.
However, if the bus does not stop and the driver of the bus refuses to stop, he is then allowed, due to necessity and fear of elapse of time before Maghrib Prayer, to perform prayer shortened on the board of the bus. This last ruling lies on the necessity rule which means that he should do his best to let him stop the bus. If he does not stop, in this case, he is allowed to offer the prayer shortened with any form on the bus.
May Allah guide us to what pleases Him!