The Muslims are required by sharee’ah to be united in starting and ending the fast – and how that may be achieved
Question

                Why are the Muslims not united with regard to the beginning and end of Ramadaan? How can that be achieved?.
Praise be to Allah.
    Undoubtedly for the Muslims to be united in starting and 
    ending the fast is something desirable and it is something that is required 
    by sharee’ah wherever possible, and there are only two conditions of 
    achieving that: 

    1 –All Muslim scholars should do away with relying on 
    calculations as the Messenger of Allaah  (peace and blessings of Allaah 
    be upon him) and the early generations of this ummah did, and follow 
    moonsighting or completing the number of days (in the month) as was 
    explained by the Messenger of Allaah  (peace and blessings of Allaah be 
    upon him) in the saheeh ahaadeeth. Shaykh al-Islam Ibn Taymiyah (may Allaah 
    have mercy on him) said in al-Fataawa (25/132, 133) that the scholars 
    were unanimously agreed that it is not permissible to rely on calculations 
    for proving the beginning and end of the fast and so on. Al-Haafiz narrated 
    in al-Fath (4/127) that al-Baaji said: The consensus of the salaf is 
    that calculations should not be relied on, and their consensus is proof for 
    those who come after them. 

    2 – They should commit to following the sighting of the moon 
    in any Muslim state that follows the sharee’ah of Allaah and adheres to its 
    rulings. When it is proven in that country that the new moon has been 
    sighted with shar’i proof, with regard to the beginning or end of the fast, 
    then they should follow that, in accordance with the words of the Prophet 
     (peace and blessings of Allaah be upon him): “Fast when you see it, and 
    stop fasting when you see it, and if it is too cloudy then complete the 
    number (of days)” and “We are an unlettered nation, we do not write or 
    calculate. The month is such-and-such” and he gestured with his hands three 
    times, and held down his thumb the third time, then he said, “or the month 
    is such-and-such” and he  gestured with all of his fingers, meaning that the 
    month may be twenty-nine days or it may be thirty. There are many ahaadeeth 
    with similar meanings, from Ibn ‘Umar, Abu Hurayrah, Hudhayfah ibn al-Yamaan 
    and others (may Allaah be pleased with them). It is well known that what the 
    Prophet  (peace and blessings of Allaah be upon him) said was not 
    addressed only to the people of Madeenah, rather it is addressed to the 
    entire ummah at all times and in all places until the Day of Resurrection. 
    When these two conditions are met, then all Muslim countries will be able to 
    fast together and break the fast together, and we ask Allaah to enable them 
    to do that and to help them to rule by Islamic sharee’ah and reject that 
    which goes against it. Undoubtedly that is obligatory upon them because 
    Allaah says (interpretation of the meaning): 

    “But no, by your Lord, they can have no Faith, until they 
    make you (O Muhammad صلى الله عليه وسلم) 
    judge in all disputes between them, and find in themselves no resistance 
    against your decisions, and accept (them) with full submission”

    [al-Nisa’ 4:65]

    And there are similar verses. 

    There is also no doubt that ruling all their affairs in 
    accordance with sharee’ah is in their best interests and will unite them and 
    help them against their enemies and help them to attain happiness in this 
    world and in the Hereafter. We ask Allaah to open their hearts to that and 
    to help them to achieve it, for He is All-hearing, Ever-near. End quote. 

    Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him).