How should a traveler pray if he is moving about for a week? Should he pray a shortened prayer (qasr) or a full prayer (itmaam)? And what should he do if he enters a mosque and finds a congregation praying Maghrib while he intends to pray Isha? Should he join them with the intention of Isha?

It is sunnah for a traveler (for a distance of no less than 81 km) to shorten the four-rak'ah prayer to two rak'ahs. It is also permissible for him to combine the Dhuhr and Asr prayers, and the Maghrib and Isha prayers, either by performing the earlier prayer first (taqdeem) or by delaying the earlier prayer to be performed with the later one (ta'kheer). If the sun declines while he is settled, it is preferable to combine Dhuhr and Asr prayers by performing the earlier prayer first (taqdeem), if he intends not to depart before the sun turns yellow (approaching sunset). If the sun declines while he is traveling, it is preferable to combine Dhuhr and Asr prayers by delaying the earlier prayer (ta'kheer). If the sun sets while he is settled, it is preferable to perform the Isha prayer with the Maghrib prayer by performing the earlier prayer first (taqdeem). If the sun sets while he is traveling, he may delay the Maghrib prayer to be performed with the Isha prayer. It is permissible for a traveler to join the Isha prayer with a congregation performing the Maghrib prayer. When the imam finishes the third rak'ah, the traveler stands up to perform his fourth rak'ah.