What is the distance which, once one has set out for it, allows one to shorten the prayer?
The majority of scholars (al-jumhoor) comprising the Maliki's and the Shafiei's and the Hanbali's have taken the opinion that the recognized distance for one who has undertaken its travel in shortening the prayer is four burud (an antiquated unit of distance), which is two average day's travel by heavily-loaded camels (equivalent to 88.7 km in distance). Among what they have quoted as evidence is what was authentically (sahih) narrated by Ibn Omar and Ibn Abbas (may Allah be pleased with them) that they used to shorten the prayers and break fasting at a distance of four BURUD. This quoted distance is approximate and not exactly limited as per the majority of scholars, and thus what is slightly less is exempted as well.
Some scholars including ibn Qadama and Sheikh ul-Islam Ibn Taymiyya and his pupil Ibn ul-Qayyim have taken the opinion that all that is referred to as travel in practice and in language, and requires preparation of provisions as well as rest and similar things, falls under the licenses of shari'a such as the shortening of the prayer and breaking the fast of Ramadan. Their pretext and justification is the generalization in the wording referring to the shortening of the prayer as it appears in the Qur'an and sunnah, as in the example of surat al-Nisaa', verses 4:101-102:
"When you travel through the earth, there is no blame on you if you shorten your prayers, for fear the unbelievers may attack you: for the unbelievers are unto you open enemies. When you (O Messenger) are with them, and stand to lead them in prayer, let one party of them stand up (in prayer) with you taking their arms with them; when they finish their prostrations, let them take their positions in the rear. And let the other party come up - which has not yet prayed - and let them pray with you, taking all precautions, and bearing arms: the Unbelievers wish, if you were careless of your arms and your baggage, to assault you in a single rush. But there is no blame on you if you put away your arms because of the inconvenience of rain or because you are ill; but take (every) precaution for yourselves. For the unbelievers Allah has prepared a humiliating punishment."
And in the following hadith:
Al-Tirmidhi, hadith 2960: Ya'la bin Umayyah said: I said to 'Umar ibn al-Khattaab "Verily Allaah has said ' if you shorten your prayers, for fear the unbelievers may attack you ' and now the people feel secure." 'Umar said, "[Indeed] I wondered the same thing you are wondering, so I mentioned it to the Prophet (peace be upon him) and he said '[It is] a charity that Allaah has bestowed upon you so accept His charity.' " (Abu 'Isa said this hadith is hasan sahih.)
Al-Tirmidhi, hadith 453: Umayyah ibn Abdullah ibn Khaalid ibn Asid said to Ibn 'Umar, "How can you shorten the prayer when Allaah the Almighty has said, 'there is no blame on you if you shorten your prayers, for fear ' ," so Ibn 'Umar said, "O son of my brother, verily the Prophet (peace be upon him) came to us when we were misguided, so he taught us, and among what he taught us was that Allaah the Almighty has ordered us to pray two raka'a during travel." (Al-Shu'aithiy said that al-Zuhriy used to relate this hadith via Abdullah ibn Abu Bakr.)
The ayah expresses the permission for shortening the prayer for one who travels without specifically restricting the distance. Thus the Qur'an and sunnah mention "travel" and do not differentiate a particular travel from another, and as such if one travels via air for one hour without any burden or hardship it would be permissible for him or her to shorten the prayer and break the mandatory fast. In fact, this is the most viable opinion, unless for a particularly case there is confusion as to whether it is commonly regarded as travel or not, in which case one falls back to the opinion of the majority of scholars (al-jumhoor) (regarding the minimum required distance).