Narrated Anas : When Allah’s Messenger (ﷺ) went out on a journey of three miles or three Farasikh, he (ﷺ) used to pray two Rak’at. [Reported by Muslim].

Farasikh (plural of Farsakh) is Persian unit of distance. Farsakh is equal to about three miles.

Shaikhul-Islām Ibn Taymiyyah stated: “The evidence supports those who say that the legislation allows the prayer to be shortened and the fast to be broken on whatever falls into the category of a journey — and they do not single out one type of journey to the exclusion of another.”
[Majmoo’ Al-Fatawa 24/106]. That is because Allah (the Most High) and His Messenger صلى الله عليه وسلم did not specify a particular distance at which one is permitted to shorten the prayers.

Shaikh Al-Albāni also agreed with the position of Ibn Taymiyyah. Al-Albāni makes the added point that the ‘Urf of the people is the best measure as it agrees with the ease that Allah has prescribed for mankind in worship.

Ibn ‘Abbās also narrated “The prayer is shortened for a journey from Makkah to ‘Usfān or to Tā’if.” And it is known that the distance between ‘Usfān and Makkah is 80km on the road to Madinah. And between Tā’if and Makkah is likewise 80km.

Ibn ‘Uthaymeen stated “The distance at which some of the scholars stated as a limit at which one can shorten the prayer is approximately 83km. Other scholars have considered the customary practice of the people [in a country] to be the measure which decides a journey even if that does not extend to 80km (or so). And that which the people (of a land) state regarding travel, “It is not a journey” then it is not a journey even if it reaches a 100km…”

And when the custom (‘urf) of the people differs between themselves, then there is no harm if a person takes the saying of those scholars who set a limit (of approximately 80km) before one is considered a traveller — because that is the saying of some of the Imāms, Scholars and Jurists.

So there is no harm in that (inshā’-Allāh). However, if the matter is clearly defined by the practice (‘urf) of the people, then it is returned back to that — and that is what is correct.”
[Fatawa Arkān Al-Islām, p. 381]

Shaikh Ibn Bāz (rahimahullāh) said, “That which the majority of the scholars hold is: That a journey is that which amounts to approximately 80 kilometres, whether by car, aeroplane or ship. This is the distance (or approximate distance) which is termed a journey. This distance is the custom that is well-known among the Muslims. So when a person travels by camel, or foot, or car, or aeroplane or ship for this distance or more, then he is a traveller.” [ Majmoo’ Al-Fatawa 12/267]