Entering the mosque when the imam is standing
Question

                If I enter the mosque when the imam is standing and reciting Qur’aan, should I say the opening du’aa’ or recite al-Faatihah straight away?.
Praise be to Allah.Whoever enters the mosque 
    when the imam is reciting al-Faatihah should say the opening takbeer, then 
    be silent until the imam has finished reciting al-Faatihah, because the 
    person who is praying behind an imam is enjoined to listen attentively to 
    the recitation of his imam, and his reciting the opening du’aa’ and seeking 
    refuge with Allaah will distract him from listening attentively as he is 
    enjoined to do. He should not let himself be distracted from that which he 
    is enjoined to do. 

    See Majmoo’ al-Fataawa, 23/280 

    His view – that he should not recite the opening du’aa’ or 
    seek refuge when his imam is reciting out loud – is the more correct of the 
    two scholarly views on this matter, in sha Allah. It is the view favoured by 
    Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) as stated in 
    Majmoo’ al-Fataawa (22/341 and 23/271), because of the strength of the 
    evidence. The Prophet  (peace and blessings of Allaah be upon him) said: 
    “The imam has been appointed to be followed, so when he says takbeer, then 
    say takbeer, and when he recites, listen attentively.” Narrated by Muslim, 
    404; al-Nasaa’i, 921. 

    When the imam has finished al-Faatihah, the person behind him 
    may say the opening du’aa’, then pray for refuge with Allaah and recite the 
    Basmalah and al-Faatihah. If he cannot recite the opening du’aa’ and prayer 
    for refuge and recite before the imam starts to recite the next soorah, then 
    he should not recite the opening du’aa’, because the opening du’aa’ is 
    Sunnah; rather he should recite the prayer for refuge and al-Faatihah, 
    because the Prophet  (peace and blessings of Allaah be upon him) said 
    with regard to reciting behind the imam: “Do not do anything but recite the 
    Essence of the Qur’aan [al-Faatihah], for there is no prayer for the one who 
    does not recite it.” Narrated by al-Tirmidhi, 311; classed as saheeh by 
    al-Albaani in Saheeh al-Tirmidhi. 
This is according to the 
    view that it is obligatory for the person who is praying behind an imam in a 
    prayer where Qur’aan is recited out loud to recite al-Faatihah. The correct 
    view is that it is obligatory for him, because of the general meaning of the 
    words of the Prophet  (peace and blessings of Allaah be upon him): 
    “There is no prayer for the one who does not recite the Opening of the 
    Book.” Narrated by al-Bukhaari, 756. This means that such a prayer is not 
    valid. 

    See also question no. 10995 

    But if he enters the mosque and the imam is offering a prayer 
    when Qur’aan is recited silently, then he should say the opening takbeer and 
    recite the opening du’aa’ and prayer for refuge, then recite al-Faatihah if 
    he thinks that he can complete it before the imam bows, so that he may earn 
    the reward for reciting the opening du’aa’ and al-Faatihah. Otherwise he 
    should focus on al-Faatihah after saying the opening Takbeer, and not recite 
    the opening du’aa’, because it is more important to pay attention to that 
    which is obligatory. 

    See Mughni al-Muhtaaj, 1/257; Talbees Iblees, 
    p. 161 
If the imam bows, the 
    person praying behind him should leave the rest of al-Faatihah and bow with 
    him, because he has only caught up with what he has recited, and he has 
    caught up with the rak’ah, as is the case if he catches up with him when he 
    is bowing. So in this case the obligation to recite al-Faatihah is waived, 
    and he should not lag behind his imam in order to complete al-Faatihah, 
    because the Prophet  (peace and blessings of Allaah be upon him) said: 
    “When he bows, then bow.” Narrated by al-Bukhaari, 378; Muslim, 414. 

    Shaykh Ibn Baaz (may Allaah have mercy on him) said: 
Reciting the opening du’aa’ is Sunnah 
    and reciting al-Faatihah is obligatory for the one who is praying behind an 
    imam, according to the correct scholarly opinion. If you fear that you will 
    miss al-Faatihah, then start with it, and if the imam bows before you have 
    completed it, then bow with him and the rest of it is waived in your case, 
    because the Prophet  (peace and blessings of Allaah be upon him) said: 
    “The imam has been appointed to be followed, so do not differ from him. When 
    he says takbeer, then say takbeer; when he bows, then bow…” Agreed upon. End 
    quote. 

    Majmoo’ Fataawa Ibn Baaz, 
    11/243-244. 

    And Allaah knows best. 

    See Ahkaam Hudoor al-Masaajid, p. 139-141, by Shaykh ‘Abd-Allaah ibn Saalih al-Fawzaan.