Praying behind the Traveller
Question: What should the resident do if he prays behind the traveller?
Response: If the resident prays behind the traveller [leading the prayer] – who has shortened his prayer, then the resident’s prayer behind him is correct; So when the [traveller] imaam completes the prayer, then the resident must stand and complete the remaining two rak’ahs of the prayer.
Shaykh ‘Abdur-Rahmaan as-Sa’dee
al-Fataawa as-Sa’diyyah – Page 126, Question 8 source: fatwaonline.com