The traveler leading the resident in prayer and the reverse

Q: If a person travels and wants to pray the midday prayer in congregation, and he finds a person who has already performed the midday prayer while being a resident, does the resident pray with the traveler? And does he shorten the prayer with him or complete it?

A: If the resident prays behind the traveler seeking the virtue of the congregation, and the resident has already prayed his obligatory prayer, then he prays like the traveler, two rak'ahs, because it is a voluntary prayer for him. However, if the resident prays behind the traveler the obligatory prayer, such as the midday, afternoon, or night prayer, then he prays four rak'ahs, and thus he is required to complete his prayer after the traveler concludes the two rak'ahs. As for if the traveler prays behind the resident the obligatory prayer for both of them, then the traveler is required to complete it as four rak'ahs in the more correct of the two opinions of the scholars.
This is because Imam Ahmad narrated in his Musnad, and Imam Muslim in his Sahih, may Allah have mercy on both of them, that Ibn Abbas was asked about the traveler who prays four rak'ahs behind the resident imam and prays two rak'ahs with his companions, and he said: "This is the Sunnah." And due to the generality of the saying of the Prophet (peace and blessings be upon him). It is agreed upon that it is authentic.