Answered by Shaykh Faraz Rabbani
If a traveller prays behind a resident imam, does he pray the same number of rakas as the imam, even if he is a latecomer to the prayer?
Walaikum assalam wa rahmatullah,
Yes: The traveller prays the same number of rakats are their imam, even if they are a latecomer. When the traveller leads or prays alone, however, they have to shorten their prayers. [Shurunbulali, Nur al-Idah]
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And Allah alone gives success.
Wassalam,
Faraz Rabbani