Reference: http://mufti.af.org.sa/ar/content/المسافر-إذا-صلى-مع-جماعة-مقيمين-،-هل-يتم-أم-يقصر
Question: [At the moment] I am travelling, so if I joined a congregation of local residents for the ‘ishaa prayer, [as a traveller] should I finish my prayer after two rak’ahs, or should I pray all four rak’ahs along with them?
Response: If the traveller joins a congregation of local residents [in prayer], then he must complete the prayer along with them.
