(9) باب إِذَا رُؤِيَ الْهِلاَلُ فِي بَلَدٍ قَبْلَ الآخَرِينَ بِلَيْلَةٍ
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.
Reference: Sunan Abi Dawud 2333
In-book reference: Book 14, Hadith 21
English translation: Book 13, Hadith 2326