Joining the imam in any position
Question

                If a worshipper enters the mosque and the imam is prostrating, should he join him in prostration or wait until he gets up?.
Praise be to Allah.
    If the worshipper enters the mosque and the imam is praying, 
    he should join him no matter what position he is in, whether he is standing, 
    bowing, prostrating or between prostrations. That is because of the report 
    narrated by Abu Hurayrah (may Allaah be pleased with him) who said: The 
    Messenger of Allaah  (peace and blessings of Allaah be upon him) said: 
    “If you come to prayer and we are prostrating, then prostrate, but do not 
    regard it as anything. Whoever catches up with a rak’ah has caught up with 
    the prayer.” Narrate by Abu Dawood, 893; classed as hasan by al-Albaani in
    Saheeh Abi Dawood. 
It was narrated that Abu 
    Qataadah (may Allaah be pleased with him) said: The Messenger of Allaah  
    (peace and blessings of Allaah be upon him) said: “When you come to prayer 
    you must be dignified. Whatever you catch up with, pray, and whatever you 
    miss, complete it.” Narrated by al-Bukhaari, 635. 

    Al-Haafiz Ibn Hajar (may Allaah be pleased with him) said in
    Fath al-Baari, 1/118: This is quoted as evidence that it is mustahabb 
    to join the imam in whatever position he is found.” End quote. 

    It was narrated that Mu’aadh ibn Jabal said: The Prophet  
    (peace and blessings of Allaah be upon him) said: “When one of you comes to 
    the prayer and the imam is in any position, let him do what the imam is 
    doing.” Narrated by al-Tirmidhi, 591; classed as saheeh by al-Albaani in 
    Saheeh al-Tirmidhi. 
This is the Sunnah with 
    regard to the one who enters the mosque and finds the imam praying, based on 
    the evidence in the texts. 

    Ibn Hazm (may Allaah have mercy on him) narrated that there 
    was scholarly consensus on this point. He said in Maraatib al-Ijmaa’ 
    (p. 25): They said: If a person comes and the imam has already done some of 
    the prayer, a little or a lot, and there is nothing left but the salaam, he 
    should join him and do the same as he finds him doing, unless he has decided 
    to catch up with the congregation in another mosque. End quote. 

    Ibn Qudaamah said in al-Mughni (2/184): 

    It is mustahabb for the one who catches up with the imam in 
    any position to follow him in that, even if it will not count as anything 
    for him… Then he quoted some of the ahaadeeth that we have quoted above… 
    Then he said: This is what is to be done according to the scholars, who 
    said: If a man comes and the imam is prostrating, let him prostrate, but 
    that rak’ah will not count for him. Some of them said: perhaps by the time 
    he raises his head, he will have been forgiven. End quote. 

    There are some people who, if they enter the mosque and find 
    that the imam is prostrating or is between the two prostrations, they do not 
    join him until he stands up for the second rak’ah or until they know that he 
    is reciting the tashahhud, then they sit with him. Such a person has 
    deprived himself of the benefit of prostration, as well as going against 
    what is indicated by the evidence quoted above. 

    It says in Tuhfat al-Ahwadhi (2/199): Let him do what 
    the imam is doing, i.e., let him join the imam whether he is standing, 
    bowing or whatever. He should not wait for the imam to stand up as the 
    common folk do. End quote. 

    See: Ahkaam Hudoor al-Masaajid, p. 138-139, by Shaykh 
    ‘Abd-Allaah ibn Saalih al-Fawzaan. 
The scholars of the 
    Standing Committee for Issuing Fatwas were asked about a man who enters the 
    mosque when the imam is in the final rak’ah. Should he join them or wait 
    until they have finished? 

    They replied: 

    What is prescribed in your situation is to join them. 
    Whatever you catch up with, pray with them, and whatever you miss, make it 
    up. If you join them after they stand up from bowing in the last rak’ah, 
    then join them and make up the entire prayer after the imam says the salaam. 
    …. Then they quoted as evidence the ahaadeeth that we have quoted above. End 
    quote. 

    ‘Abd al-‘Azeez ibn ‘Abd-Allaah ibn Baaz, ‘Abd al-Razzaaq 
    ‘Afeefi, ‘Abd-Allaah ibn Qa’ood, ‘Abd-Allaah ibn Ghadyaan. 

    The scholars of the Standing Committee were also asked: What 
    is better if a man enters the mosque and the imam is reciting the final 
    tashahhud – should he join him in the tashahhud or is it better to wait for 
    some other people to come so that they can pray together? 

    They replied: 

    It is better for him to join the imam, because of the general 
    meaning of the hadeeth: “Whatever you catch up with, pray, and whatever you 
    miss, make it up.” End quote. 

    ‘Abd al-‘Azeez ibn ‘Abd-Allaah ibn Baaz, ‘Abd al-Razzaaq 
    ‘Afeefi, ‘Abd-Allaah ibn Qa’ood. 

    Fataawa al-Lajnah al-Daa’imah, 
    7/312-323.