Reference: http://www.alfawzan.af.org.sa/node/14750
Question: Is it permissible to combine [the prayer] during his stay in the city he has reached?
Response: [The ruling regarding] this is dependant upon [the duration of] his stay. If he does not know how long he intends to remain there for the purpose of his stay, and thereby does not know when he will finish – [in this case] he shortens [his prayer]; if he intends to remain there for four days or less, likewise he shortens [his prayer].
If, however, he remains or intends to remain there for more than four days, [in this case] he is required to pray in full the moment he reaches his destination, because [by reaching his destination] he is now considered a resident, and no longer afforded the ruling of a traveller.
