A person who is a Musaafir joins the Imaam and did not know the Imaam was also a Musaafir. When the Imaam made Salaam after two Rakaats of Dhuhr Salaat, the follower stands up and completes four Rakaats.
If the Muqtadi (follower) is a Musaafir and he is sure that the Imaam is also a Musaafir, then he should terminate his Salaat with the Imaam. If the Muqtadi is in doubt as to whether the Imaam is a Musaafir or not, he should complete his Salaat with the Imaam and thereafter ascertain the position of the Imaam being a Musaafir or not. If he was a Musaafir, the two Rakaats Salaat behind the Imaam will be valid. If the Imaam was a Muqeem, then the Muqtadi’s Salaat will be invalid. The Muqtadi will have to repeat the Salaat. (Imdaadul Ahkaam vol.1 pg.723)
Al-Mahmood 18