Local or Global Moon Sighting ?

Wa alaykum as salaam!

Thank you for your question! As for the ruling regarding fasting based on local sightings vs. international sightings, local sightings is preferred. This is based upon the narration of Kurayb:

I arrived in Greater Syria, and fufilled a need for her [i.e. Umm al-Fadl]. It was there in Greater Syria that the month of Ramadan began. I saw the new moon on Friday. I then came back to Medina at the end of the month. Abdullah b. 'Abbas asked me: “When did you see the new moon?” I replied: “We saw it on Friday night.” He asked: “Did you see it yourself?” I said: “Yes, and the people also saw it and they fasted and Mu'awiya also fasted. He said: “But we saw it on Saturday night. So we would continue to observe fast till we complete thirty or we see it [i.e. the new moon of Shawwal].” I said: “Is the sighting of the moon by Mu'awiya not valid for you?” He said: “No; this is how the Allah’s Messenger, peace and blessings be upon him and his progeny, has commanded us.”
Yahya b. Yahya doubted whether the wording used was “valid for us” or “valid for you.”

This report is very explicit in its denotation. There's no need for commentary.

There is also the narration narrated in Al-Jaami al-Kaafi in which Muhammad ibn al-Mansur (ra) narrated with a complete chain of narrators to A'isha:

I said "O Messenger of Allah, the people of this city are beginning the fast today and the people of another city are beginning the fast tomorrow. The people of this city are breaking their fast today and the people of another city are breaking their fast tomorrow. The people of this city are sacrificing today and the people of another city are sacrificing tomorrow. This is contradiction!"
Allah's Messenger, peace and blessings be upon him and his progeny, said: ((This is not contradiction. Rather, the fast of the people who fast, the breakfast of the people who break fast and the sacrifice of the people who sacrifice are a mercy from Him)).

The second narration is instructive because:
1.) It shows that even during the time of the Prophet, peace and blessings be upon him and his progeny, the Muslims fasted and broke fast based on locale and not international sightings.
2.) He did not censure such action nor did he enforce the fast and breaking fast on what they were doing in Medina.
3.) He saw it as a mercy and not a contradiction as many Muslims today tend to do.

And Allah knows best!

IRS