Do you have to start fasting as soon as any Muslim country starts fasting or only when the moon is sighted in your country or city? If it is the latter which applies, is it by country or city? Please cite the sources you rely on.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.

There is no doubt that the time of sighting the new moon can differ from one region to another and this is a well-known fact. No Muslim scholar or layman could disagree with this. The difference of opinion among the scholars lies in considering the time differences of sighting the new moon across many countries. Do all Muslims have to start the fast, or break it if the moon has been seen in any place anywhere in the world, or should they just stick to the sighting of the country where they actually are? This issue is among those in which Ijtihaad (scholarly deduction) may be done and Muslim scholars have two opinions in this regard:

A – If the new moon is seen anywhere in the world, then any Muslim anywhere must begin fasting accordingly. The difference in the time of sighting is not considered. This is so because the Prophet said: “When you see the new moon, observe fasting, and when you see it (again) then break fasting (i.e. at the end of the month), and if the sky is cloudy for you, then complete thirty days for the month of Sha’baan.” [Al-Bukhari and Muslim]

B -The other scholars take into consideration the time difference in sighting the new moon. Consequently, they say that every country has its particular sighting of the new moon. The people of a particular region should fast when they see it. Their evidence is also the same Hadeeth mentioned above. They say that this Hadeeth should not be generalized for all Muslims. They also cite another Hadeeth narrated by Kurayb that Ibn ‘Abbaas, may Allaah be pleased with him, said, “But we would continue to observe fasting till we completed thirty (days) or we saw the new moon.” [Muslim] In this case, the decision of the Islamic authorities in your country should be followed. They may choose one of the above opinions. There is no hardship in this matter and Muslims should not make it an issue of conflict and disagreement. We believe that everyone should follow the decision of the Islamic Centers in their own country, so that Muslims may maintain their unity in their countries.

Allah Knows best.