Should he fast with his own country or with any country that sights the new moon?

 

those who have seen the
new moon may be trusted by others who follow their sighting, or they may not
be trusted, and hence differences arise. One country may sight it and rule
accordingly, and fast and break the fast on that basis, whereas another
country may not be convinced of this sighting and may not trust the other
country, for many reasons, political and otherwise. 

What the Muslims must do is fast all together when they see
the new moon, and break the fast when they see it, because of the general
meaning of the words of the Prophet (peace and blessings of Allaah be
upon him): “When you see the new moon then fast, and when you see the new
moon then break the fast, and if it is too cloudy then complete the number
of days as thirty.” If they are all certain of the validity of the moon
sighting, and that it is real and proven, then they must fast and break the
fast accordingly, but if the people differ and do not trust one another,
then you have to fast with the Muslims in your country, and break the fast
with them, in accordance with the words of the Prophet (peace and
blessings of Allaah be upon him):  “The fast is the day when you fast, and
the breaking of the fast (al-iftaar) is the day when you break the fast, and
the sacrifice (al-adha) is the day when you offer the sacrifice.” 

It was proven from Ibn ‘Abbaas (may Allaah be pleased with
him) that when Kurayb told him that the people of Syria had started fasting
on a Friday, Ibn ‘Abbaas said: We saw it on Saturday, and we will carry on
fasting until we see the new moon or we complete thirty days. He did not
follow the sighting of the people of Syria because Syria is far away from
Madeenah and there may be a difference in moonsighting between the two. He
(may Allaah be pleased with him) thought that this was a matter that was
subject to ijtihaad. You have a good example to follow in Ibn ‘Abbaas and
the scholars who followed him and said that you should fast with the people
of your country and break the fast with them. And Allaah is the Source of
strength. End quote. 

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