Narrated Kuraib: That Umm al-Fadl, daughter of al-Harith, sent him to Mu'aqiyah in Syria. He said: I came to syria and performed her work. The moon of Ramadan appeared while I was in Syria. We sighted the moon on the night of Friday. When I came to Median towards the end of the month (of Ramadan), Ibn Abbas asked me about the moon. He said: When did you sight the moon ? I said: I sighted it on the night of Friday. He asked: Did you sight it yourself ? I said: Yes, and the people sighted it. They fasted and Muawiyah also fasted. He said: But we sighted it on the night of saturday. Since then we have been fasting until we complete thirty days or we sight it. Then I said: Are the sighting of the moon by Muawiyah and his fasts not sufficient for us? He replied: No. The Messenger of Allah ﷺ commanded us to do so.
Hadith Referenceسنن ابي داود / كتاب الصيام / 2332
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:صحيح مسلم (1087)
Al-Hasan said about a person who was in a certain city. He fasted on Monday, and twp persons bore witness that they had sighted the moon on the night of sunday. He said: That man and the people of his city should not fast as an atonement except that they know (for certain) that the people of a certain city of Muslims had fasted on Sunday. In that case they should keep fast as an atonement.
Hadith Referenceسنن ابي داود / كتاب الصيام / 2333
Hadith Gradingالألبانی:صحيح مقطوع | زبیر علی زئی:إسناده صحيح