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”
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).