Permissibility of following the imaam from behind the door of the mosque
Question

                The nature of the place in which we pray is such that when it gets full, we close the door, and some people pray outside the prayer-place, following the imaam by hearing his voice on the loudspeakers. Is their prayer valid even though they are outside the mosque?
Praise be to Allah.It 
          was narrated that Zayd ibn Thaabit (may Allaah be pleased with him) 
          said: The Messenger of Allaah  (peace 
          and blessings of Allaah be upon him) was staying in a hut made of palm 
          leaves. He prayed in that hut and some men followed him, they came and 
          prayed with him. And in the hadeeth in which this incident is described, 
          it says: “The best prayer of a man is prayers offered in his house, 
          except for the obligatory prayers.” (Agreed upon) 
 This 
          hadeeth indicates that it is permissible to follow the imaam even if 
          he is in a room where the person following him cannot see him, or if 
          one of them is on the roof and the other is in a lower place. The point 
          is that if it is possible to follow the imaam if they are all inside 
          the mosque, there is no dispute among the scholars that it is permissible 
          for him to do so. 
 The 
          hadeeth also indicates that the presence of a barrier between the imaam 
          and the members of the congregation does not mean that the prayer and 
          their following the imaam are invalid. Al-Nawawi said: “For their following 
          to be valid, the person following has to know when the imaam moves, 
          whether they are praying in the mosque or elsewhere, or one of them 
          is in the mosque and the other is outside – this is according to scholarly 
          consensus.” 
 If 
          one of them is outside the mosque and he can see the imaam or other 
          members of the congregation, even if the rows are interrupted, the prayer 
          is valid because there is nothing to make it invalid and there is something 
          to make it valid – which is that he can see either the imaam or some 
          of the people behind him, and he is able to follow the prayer. 
 In 
          al-Insaaf it says: with 
          regard to the continuity of the rows, reference should be made to al-‘urf 
          (local custom, i.e., what is traditionally regarded as being complete), 
          according to the correct view. 
 It 
          says in al-Mughni: interruptions 
          (in the rows) do not matter. This is the view of Maalik and al-Shaafa'i, 
          because there is no limit in that, since it does not prevent following 
          the imaam. What matters here is what would prevent a person from seeing 
          or hearing the imaam. Al-Nawawi made it a condition that there should 
          not be big gaps outside the mosque, which is the view of the majority 
          of scholars. And Allaah knows best.