If a fasting person travels from one Islamic country to another, and it was announced in the first country that the new crescent of Shawwal has been sighted, should he follow this announcement and break his fast, even if the new crescent has not been sighted in the second country?
The answer depends on the direction of his travel and the timing of the sighting in each country.
If a person travels to a country where the breaking of the fast is delayed, he should continue fasting with the people there until they also break their fast. This aligns with the principle that fasting begins when the people start fasting and ends when they break their fast. Likewise, Eid al-Adha (the day of sacrifice) is observed on the day when the people sacrifice their animals, even if this means his fasting is extended by a day or more. This situation is similar to traveling to a place where sunset occurs later than his usual time, resulting in a longer day by several hours or more.
In contrast, if he travels from a country that has delayed confirming the start of the month to one that has already confirmed it, he should break his fast with the people in the new country and then make up any missed days of Ramadan after Eid. For instance, if he missed one day, he should make up one day, and if he missed two, he should make up two days. This approach is necessary because it is not possible for the month to be less than 29 days or more than 30 days. Thus, he should break his fast upon sighting the new crescent, even if he hasn’t completed 29 days. Since he fasted fewer than 29 days, he must make up for the incomplete days.
The Prophet ﷺ instructed, "Break your fast when you see it," meaning the new crescent, so if it has not been sighted in the second country, he remains in Ramadan and should not break his fast. This temporary extension of fasting can be considered similar to the natural extension of daylight when one travels westward.
(Source: Fataawa Arkaanul Islam by Shaykh Ibn Uthaymeen)