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 Mu'awiyah 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 Mu'awiyah and his fasts not sufficient for us? He replied: No. The Messenger of Allah ﷺ commanded us to do so.  

أَنَّ أُمَّ الْفَضْلِ ابْنَةَ الْحَارِثِ بَعَثَتْهُ إِلَى مُعَاوِيَةَ بِالشَّامِ قَالَ فَقَدِمْتُ الشَّامَ فَقَضَيْتُ حَاجَتَهَا فَاسْتُهِلَّ رَمَضَانُ وَأَنَا بِالشَّامِ فَرَأَيْنَا الْهِلاَلَ لَيْلَةَ الْجُمُعَةِ ثُمَّ قَدِمْتُ الْمَدِينَةَ فِي آخِرِ الشَّهْرِ فَسَأَلَنِي ابْنُ عَبَّاسٍ ثُمَّ ذَكَرَ الْهِلاَلَ فَقَالَ مَتَى رَأَيْتُمُ الْهِلاَلَ قُلْتُ رَأَيْتُهُ لَيْلَةَ الْجُمُعَةِ قَالَ أَنْتَ رَأَيْتَهُ قُلْتُ نَعَمْ وَرَآهُ النَّاسُ وَصَامُوا وَصَامَ مُعَاوِيَةُ قَالَ لَكِنَّا رَأَيْنَاهُ لَيْلَةَ السَّبْتِ فَلاَ نَزَالُ نَصُومُهُ حَتَّى نُكْمِلَ الثَّلاَثِينَ أَوْ نَرَاهُ فَقُلْتُ أَفَلاَ تَكْتَفِي بِرُؤْيَةِ مُعَاوِيَةَ وَصِيَامِهِ قَالَ لاَ هَكَذَا أَمَرَنَا رَسُولُ اللَّهِ ﷺ  

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