If he travels during Ramadaan to another country that started the fast at a different time, how should he fast?

If a man travels from one country to another where the
moonsighting was different, the basic principle is that he should start and
end the fast according to the country he is in when it is established that
the month has ended. But if that means that he has fasted for less than
twenty-nine days, then he has to complete the number, because the lunar
month cannot be less than twenty-nine days. This principle is based on the
words of the Prophet (peace and blessings of Allaah be upon him): “When
you see it (the new moon) then fast, and when you see it then break the
fast,” and “The month is twenty-nine days, so do not fast until you see it,
and do not stop fasting until you see it.” And in the hadeeth of Kurayb is
says that Umm al-Fadl sent him to Mu’aawiyah in Syria, and Kurayb told Ibn
‘Abbaas (may Allaah be pleased with him) that the people had seen the new
moon of Ramadaan on Friday night in Syria. Ibn ‘Abbaas said: “But we saw it
on Saturday night, so we will carry on fasting until we complete thirty days
or until we see it.” Kurayb said: “Is not the sighting and fasting of
Mu’aawiyah sufficient for you?” He said: “No; this is what the Messenger of
Allaah (peace and blessings of Allaah be upon him) enjoined upon us.” 

There follow some examples which explain this principle: 

1 – He moved from a land where the people started fasting on
Sunday to a land where the people started fasting on Saturday, and they
broke the fast on Sunday after twenty-nine days of fasting. He should break
the fast with them but he has to make up one day later on. 

2 – He moved from a land where the people started fasting on
Sunday to a land where the people started fasting on Monday. They stopped
fasting on a Wednesday after thirty days of fasting. He should carry on
fasting with them even if that means he does more than thirty days, because
he is in a place where the new moon has not been sighted, so it is not
permissible for him to break the fast. This is similar to the situation if
he traveled whilst fasting from a place where the sun sets at 6 p.m. to a
land where the sun does not set until 7 p.m. – he should not break his fast
until the sun sets at 7 p.m., because Allaah says (interpretation of the
meaning): 

“then complete your Sawm (fast) till the nightfall. And do
not have sexual relations with them (your wives) while you are in I‘tikaaf
(i.e. confining oneself in a mosque for prayers and invocations leaving the
worldly activities) in the mosques. These are the limits (set) by Allaah, so
approach them not. Thus does Allaah make clear His Ayaat (proofs, evidences,
lessons, signs, revelations, verses, laws, legal and illegal things,
Allaah’s set limits, orders, etc.) to mankind that they may become
Al-Muttaqoon (the pious)”

[al-Baqarah 2:187]

3 – He moved from a land where the people started fasting on
Sunday to a land where the people started fasting on Monday, and they
stopped fasting on Tuesday after twenty-nine days. He should break the fast
with them and their fasting will have been twenty-nine days while his
fasting will have been thirty days. 

4 – He moved from a land where the people started fasting on
Sunday and stopped fasting on Tuesday after thirty days, to a land where the
people started fasting on Sunday and stopped fasting on Monday after
twenty-nine days. He should break the fast with them, and he does not have
to make up one day, because he has completed twenty-nine days. 

The evidence that he has to break the fast in the first
example is that the new moon has been sighted, and the Prophet (peace
and blessings of Allaah be upon him) said, “When you see it, stop fasting.”
The evidence that he has to make up one day is the words of the Prophet
(peace and blessings of Allaah be upon him): “The month is twenty-nine
days,” so it cannot be less than twenty-nine days. 

The evidence that he may continue to fast for more than
thirty days in the second example is the words of the Prophet (peace
and blessings of Allaah be upon him), “When you see it, then stop fasting.”
So ending the fast is connected to the sighting of the moon, and if it is
not sighted then that day is Ramadaan in that place, so it is not
permissible for him to break the fast. 

The ruling in the third and fourth examples is obvious. 

This is what we think is
the ruling with regard to this issue, and it is based on the most correct
view, which is that the ruling varies according to the moonsighting. As for
the view that the ruling does not vary according to the moonsighting, and
that when the moon is sighted according to the shar’i conditions in one place, all the people must start or end the fast, this ruling depends on the evidence of the sighting of the moon, but he has to start fasting or stop fasting discreetly so that he does appear to be going against the community.  End quote.