While travelling, It is acceptable to shorten and combine the salah and it is mentioned both in the Quran and is a confirmed Sunnah of Prophet Muhammad (peace and blessings upon him).

How Far Does One Have To Travel To Pray Shortened Prayer?

There is no hadith or mention in the Quran on how far one must travel that specifies the need for Qasr Salah. The jurists argue, the minimum required distance should approximately 55 miles (around 88.7km). This is around four burud (an antiquated unit of distance representing two days of travel with a Camel).

The shortening of the prayer can also commence once he has left his village or city.

How Many Days Can You Offer the Shortened Salah?

There is a difference of opinion as to how many days you can pray the shortened salah as this is not specified in the hadith. According to the Hanafi Fiqh, it’s permissible for a Muslim to pray shortened prayer if he plans to stay less than 15 days. Beyond 15 days he is believed to be a resident and should offer the regular fardh rakats.

On Combining the Prayers

Most jurists are in agreement that the person travelling can combine the prayers. Specificially, Zuhr(2) and Asr(2) can be combined and Maghrib(3) and Isha(2) can be combined.

There are two options when combining the prayers, you can either advance Asr and pray at the time of Zuhr or pray Zuhr delayed at the time of Asr. Same for Maghrib and Isha. The prayers you can not combine is Fajr with Zuhr, Asr with Maghrib or Isha with Fajr.

Praying In Congregation

It is still recommended that the traveller should pray in congregation. If he attends a local masjid he must follow the imam and offer the full salah.

References:

[1] It was narrated that ‘Umar said: “The prayer while traveling is two Rak’ah, and Friday is two Rak’ah, and ‘Eid is two Rak’ah. They are complete and are not shortened, as told by Muhammad (ﷺ).” Sunan Ibn Majah Grade: Sahih (Darussalam)

[2] Narrated Anas: We stayed (in Mecca) for ten days along with the Prophet (ﷺ) and used to offer shortened prayers (i.e. journey prayers). Sahih Al-Bukhari

[3] Narrated Ibn Abbas: The Prophet (ﷺ) once stayed for nineteen days and prayed shortened prayers. So when we travel led (and stayed) for nineteen days, we used to shorten the prayer but if we traveled (and stayed) for a longer period we used to offer the full prayer. Sahih Al-Bukhari

[4] It was narrated that Ya’la bin Umayyah said: “I said to ‘Umar bin Al-Khattab: ‘There is no sin on you if you shorten salah and if you fear that the disbelievers may put you in trial (attack you). But now the people are safe.’ ‘Umar said: ‘I wondered the same thing, so I asked the Messenger of Allah (ﷺ) about that and he said: This is a favor from Allah (SWT) to you, so accept His favor.” Grade: Sahih (Darussalam) Sunan an-Nasa’i

[5] It was narrated from Ibn ‘Umar that if the Messenger of Allah (ﷺ) was in a hurry to travel, he would combine Maghrib and ‘Isha’. Sunan An Nasai Grade: Sahih (Darussalam)

[6] It was narrated from Mu’adh bin Jabal that the Prophet (ﷺ) combined the Zuhr and ‘Asr, and the Maghrib and ‘Isha’ when traveling during the campaign of Tabuk. Sunan Ibn Majah Grade: Sahih (Darussalam)