A person was not
    sure whether he was in front of the imaam. What is the ruling?
Question

                If a member of the congregation is not
      sure whether the place where he was standing was in front of the imaam or not, what is the
      ruling in this case?
Praise be to Allah.His prayer is valid, as was stated by al-Shaafa’i.
    This applies whether he was further ahead of the imaam or behind him.
From Fatawa al-Imam al-Nawawi, p. 58.
But it is not permissible to deliberately stand in a place in front of
    the imaam. And Allaah knows best.