Hadith 14:21
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.
حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ، حَدَّثَنِي أَبِي، حَدَّثَنَا الأَشْعَثُ، عَنِ الْحَسَنِفِي رَجُلٍ كَانَ بِمِصْرٍ مِنَ الأَمْصَارِ فَصَامَ يَوْمَ الاِثْنَيْنِ وَشَهِدَ رَجُلاَنِ أَنَّهُمَا رَأَيَا الْهِلاَلَ لَيْلَةَ الأَحَدِ فَقَالَ لاَ يَقْضِي ذَلِكَ الْيَوْمَ الرَّجُلُ وَلاَ أَهْلُ مِصْرِهِ إِلاَّ أَنْ يَعْلَمُوا أَنَّ أَهْلَ مِصْرٍ مِنْ أَمْصَارِ الْمُسْلِمِينَ قَدْ صَامُوا يَوْمَ الأَحَدِ فَيَقْضُونَهُ
Original Reference: Sunan Abi Dawud 2333