The ruling on a traveler praying behind a resident imam, or the reverse

Q: This is a letter from the questioner, A. M., from Riyadh, who says: What is the ruling on a traveler praying behind a resident imam, or the reverse, and how should they both proceed?

A: If a resident prays behind a traveler, then when the imam concludes his prayer of two rak'ahs, the resident stands up and completes four rak'ahs. This is if the imam is the traveler, in which case he prays two rak'ahs, and then when he concludes, those behind him who are residents and not travelers stand up and pray four rak'ahs, while the travelers conclude with him. This is if the imam is the traveler. As for if the imam is the resident and the travelers are behind him, they complete the prayer with him; they do not shorten it, but rather they complete four rak'ahs. This is based on what is established in the authentic hadith from Ibn Abbas (may Allah be pleased with him and his father), who was asked about one who prays behind the imam. They said to him: O Ibn Abbas, why is it that when we pray behind the imam we pray four rak'ahs, and when we pray in our camps we pray two? He said: This is the Sunnah. Imam Ahmad narrated it in his Musnad with a good chain, and its original source is in Sahih Muslim.
This indicates that the prayer of a traveler behind a resident imam must be completed as four rak'ahs, due to the aforementioned hadith. And success is from Allah.