Allah says that there is no blame on you if you shorten the prayer while travelling (4:101). The four-rak'ah prayers (Dhuhr, Asr, Isha) are prayed as two. Aisha reported that the prayer was first prescribed as two rak'ahs, and the prayer of the traveller stayed as it was while the prayer of the resident was made four.
1Shortening (qasr)
Dhuhr, Asr and Isha are prayed as two rak'ahs. Fajr and Maghrib are not shortened.
It begins when the traveller leaves the town and continues until he returns.
A traveller who prays behind a resident imam completes the full prayer with him.
2Combining (jam')
The Prophet ﷺ combined Dhuhr with Asr and Maghrib with Isha on journeys, in the time of either prayer. The Maliki, Shafi'i and Hanbali schools allow it for the traveller. The Hanafi school allows true combining only at Arafah and Muzdalifah, and says that elsewhere the prayers may be put close together (praying the first at the end of its time and the second at the start of its own).
3Distance and duration
The distance for shortening is about 80 kilometres (48 miles) according to the majority (the Shafi'i, Maliki, Hanbali schools) and about 77 kilometres in the Hanafi school.
The duration of stay: the Hanafi school says if he intends to stay 15 days or more he prays in full; the Maliki and Shafi'i schools say four days; the Hanbali school says more than four days. Some scholars hold that the traveller may shorten as long as he is a traveller.
Where the schools differ
The four Sunni schools agree on the main ruling above and differ in these points. No school is preferred here.
Whether shortening is an obligation or a concession. the Hanafi school says it is an obligation, and the other schools say it is a concession (but preferred).
Und wenn ihr im Land umherreist, so ist es keine Sünde für euch, das Gebet abzukürzen, wenn ihr befürchtet, diejenigen, die ungläubig sind, könnten euch überfallen. Die Ungläubigen sind euch ja ein deutlicher Feind.
From the Sunnah
Sahih al-Bukhari 1090Authentic collection
The point of the hadith: Aisha said that the prayer was first prescribed as two rak'ahs, and the travel prayer remained so while the prayer at home was increased.
This is general information, not a fatwa. For your own situation, ask a qualified scholar.
Fiqh questions
The prayer of a traveller: shortening and combining
Allah says that there is no blame on you if you shorten the prayer while travelling (4:101). The four-rak'ah prayers (Dhuhr, Asr, Isha) are prayed as two. Aisha reported that the prayer was first prescribed as two rak'ahs, and the prayer of the traveller stayed as it was while the prayer of the resident was made four.
Is this a fatwa?
No. It is a summary of well-known rulings with their evidence. For your own case, ask a qualified scholar.
What if my school differs from this answer?
Follow the view of the school you follow and the scholar who teaches you. The page lists the differences of the four schools.