Validity of prayer when a traveling Imam leads residents in completing their prayer to full length (Itaam)?
Chapter on the Traveler's Prayer
Al-Mughni
Book of Prayer
Primary text
When a traveler leads residents in prayer and completes the prayer (Itaam), the prayer of the residents is complete and valid. This opinion is held by Al-Shafi'i and Ishaq. The basis for this ruling is that completing the prayer becomes obligatory upon the traveler due to their intention (niyyah), making the entire prayer obligatory for them. Furthermore, if a resident praying an obligatory prayer follows one who is praying a voluntary prayer, the prayer remains valid, as established previously.
Supporting text
Abu Hanifa, Al-Thawri, and a narration from Ahmad state that the prayer of the residents becomes invalid, while the prayer of the traveler Imam and those accompanying him remains valid. The rationale provided by the judge (Al-Qadi) is that the final two rak'ahs performed by the traveler are voluntary (nafl) from the perspective of the residents, and thus, one performing an obligatory prayer cannot follow an Imam who is performing voluntary portions of the prayer.