Joining the imam immediately in whatever position he is 
in
Question

                If a man enters the masjid while the imam (leader of prayer) is praying. 
Should he join the imam immediately in whatever position he is in and 
start praying, or should he wait to find out whether the imam is going to 
sit or stand?
Praise be to Allah.The correct answer is the one to which the evidence points. And 
  that is that he should enter with the imam no matter what part of prayer 
  the imam is insujood (prostration), qiyaam (standing), 
  rukuu' (bowing) or qu'ood (sitting). The evidence is the hadith 
  of Abi Huraira (may Allah be pleased with him), in which he said, "The 
  Messenger of Allah  (peace be upon him) said, 'If 
  you come to prayer and we are prostrating, then you should prostrate [but] dont 
  count it, and whoever catches a rakaa, then he has caught the 
  salaat'."1 
  And Muath said that [the Prophet]  (peace 
  be upon him) has said "If one of you comes to prayer and the Imam is in 
  a certain position, then do as the Imam is doing".2 
  Also, the general meaning of his saying (peace be upon him), applies: "What 
  you catch up with, pray."
From the book What Should You Do in 
  the Following Situations... ?
1 Sunan Abu Dawood 893 
  and Sahih Sunan Abu Dawood 792.2 Sunan Al-Tirmidhi 591 
  and Sahih Sunan Al-Tirmidhi 484..