Kurayb narrates that Umm al-Fadl sent him to Mu‘āwiyah in Sham for something. Kurayb says,”I went to Sham and finished the job. I was in Sham when the month of Ramadān began and we saw the moon the night of Friday. When I reached Madinah at the end of the blessed month, Ibn ‘Abbas asked me about Sham. (After answering him) He then asked me when we saw the moon.I said, ‘We saw the moon the night of Jumu‘ah.’He asked, ‘Did you see it as well?’I said, ‘Yes, I saw it too and many others saw it and we all kept fasting and so did Mu‘āwiyah (that is according to that moonsighting).’Ibn ‘Abbas said, ‘But we saw the moon on Saturday night, therefore, we will keep fasting for thirty days according to that unless we sight the moon on the 29th.’I asked, ‘You don’t think the moonsighting of Mu‘āwiyah and his fasting is enough for you?’Ibn ‘Abbas replied, ‘No, this is how the Blessed Prophet (صلى الله عليه وسلم) taught us.’”(Muslim H.1819, Tirmiðī H.629, Nasa’i H.2084, Abū-Dāwūd H.1985, Musnad Ahmad H.2653 Dar-qutni H.2234)
This hadith is clear evidence. When Ibn ‘Abbas was informed by the person who himself sighted the moon that the people of Sham had begun Ramadān one day before, he responded that it was not only his opinion, but also the teachings of the Blessed Prophet (صلى الله عليه وسلم) that the moonsighting of another locality was not compulsory upon them.