Every person who is resident in a country is obliged to start and end the fast with its people

What appears to be the case, based on the shar’i evidence, is that each person who is living in a country should start the fast with its people, because the Prophet (blessings and peace of Allah be upon him) said: “The start of the fast is the day you start and the end of the fast is the day you end it, and the day of sacrifice is the day you offer your sacrifice.” And because it is known in Islam that we are commanded to be united and warned against division and differences, and because moon sightings may differ from one region to another, according to the consensus of the scholars, as Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said. Based on that, the embassy employee in Pakistan who starts fasting with the Pakistanis is closer to the right way than the one who starts his fast with Saudi, because of the distance between the two countries and the difference between moon sightings in them. Undoubtedly when the Muslims begin and end the fast when the moon is sighted or when 30 days of the month have been completed in any Muslim country, then they are closer to the apparent meaning of the shar’i evidence. But if that is not possible, then the closest is what we have mentioned above. And Allah, may He be glorified, is the source of strength. End quote. 

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

Majmoo’ Fataawa wa Maqaalaat Mutanawwi’ah (15/98, 99) 

Shaykh Ibn Baaz was also asked: Sighting of the new moon of Ramadan and Shawwaal may be two days later in Pakistan than it is in Saudi. Should they fast with Saudi or with Pakistan? 

He replied: 

What seems to be the case to us is that you have to fast with the Muslims where you are, for two reasons. The first is that the Prophet (blessings and peace of Allah be upon him) said: “The start of the fast is the day you start and the end of the fast is the day you end it, and the day of sacrifice is the day you offer your sacrifice.” (narrated by Abu Dawood and others with a hasan isnaad), so you and your brothers, so long as you are staying in Pakistan, should fast with them when they fast and break the fast with them when they break the fast, because you are included in these words and because the moon sighting varies from one place to another. Some of the scholars, including Ibn ‘Abbaas (may Allah be pleased with him) were of the view that the people of each land have their own moon sighting. 

The other reason is that if you differ from the Muslims where you are with regard to the beginning and end of the fast, that will cause confusion, provoke questions and objections, and stir up disputes and arguments. Islam came to encourage agreement, harmony and cooperation in righteousness and piety, and to encourage people to avoid fighting and arguing, because Allah says (interpretation of the meaning): “And hold fast, all of you together, to the Rope of Allaah (i.e. this Qur’aan), and be not divided among yourselves” [Aal ‘Imraan 3:103]. 

And the Prophet (blessings and peace of Allah be upon him) said, when he sent Mu’aadh and Abu Moosa to Yemen: “Give glad tidings and do not put people off, be in harmony and do not differ.” End quote. 

Majmoo’ Fataawa wa Maqaalaat Mutanawwi’ah (15/103, 104).