Sahih Muslim Book 006, Hadith Number 2391.

Chapter : There is a sighting of the moon for every town; the sighting at one town cannot be held valid for the other town situated at a considerable distance from it.

Kuraib reported that Umm Fadl, daughter of Harith, sent him (Fadl, i.e. her son) to Mu'awiya in Syria. I (Fadl) arrived in Syria, and did the needful for her. It was there in Syria that the month of Ramadan commenced. I saw the new moon (of Ramadan) on Friday. I then came back to Medina at the end of the month. Abdullah b. 'Abbas (Allah be pleased with him) asked me (about the new moon of Ramadan) and said: When did you see it? I said: We saw it on the night of Friday. He said: (Did) you see it yourself? I said: Yes, and the people also saw it and they observed fast and Mu'awiya also observed fast, whereupon he said: But we saw it on Saturday night. So we would continue to observe fast till we complete thirty (lasts) or we see it (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 Messenger of Allah (may peace be upon him) has commanded us. Yahya b. Yahya was in doubt (whether the word used in the narration by Kuraib) was Naktafi or Taktafi.