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.
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