Is it permissible for a person to pray sometimes as resident, i.e. as if he is at home, not shortening or joining of prayers; and to shorten and join prayers together at other times? Or should he be obliged with one opinion only?.
With regard to shortening prayers when travelling, this is a confirmed Sunnah (Sunnah mu’akkadah) which should not be forsaken. The Prophet (peace and blessings of Allaah be upon him) shortened prayers during all his journeys, and it is not proven that he offered the prayers in full whilst travelling.
This has been discussed in the answer to question no. 111894.
But praying in congregation is obligatory for the traveller just as it is obligatory for the non-traveller, and he should not forsake the jamaa’ah (congregation) in order to shorten his prayer, rather he should pray with the congregation, and if the imam offers the prayer in full, then he should offer the prayer in full behind him.
With regard to the traveller joining two prayers, this is permissible, but it is better not to join them unless there is some difficulty in offering each prayer at its own time.
Based on that, the ruling on shortening and joining prayers for the traveller is not one and the same. Shortening the prayers is a confirmed Sunnah for all travellers, whereas joining the prayers is permissible but is not Sunnah, but it may be Sunnah and mustahabb if there is some difficulty involved in not doing it.
In the Sunnah, this is indicated by the fact that the Prophet (peace and blessings of Allaah be upon him) shortened his prayers on all his journeys, which indicates that the Sunnah for the traveller is to shorten his prayers. As for joining prayers, it is proven that he (peace and blessings of Allaah be upon him) joined prayers when travelling, and it is also proven that he did not join prayers, which indicates that joining prayers is not a recommended Sunnah for all travellers.
To sum up:
The traveller does not have the choice between shortening or not shortening his prayers; rather it is confirmed that he should shorten his prayers, unless he is praying behind an imam who is offering the prayer in full, in which case he should offer the prayer in full behind him.
As for joining prayers, the traveller has the choice: he may join them or not join them, or he may join them sometimes and not at other times. It is permissible but is not mustahabb, but joining prayers is mustahabb for the traveller if he needs to do that.
And Allaah knows best.