Each land has its own sighting of the moon, and if they see the crescent in one land, that does not necessarily apply to regions that are far away from it

Kuraib reported that Umm Fadl, daughter of Harith, sent him (Fadl, i. e. her son) to Mu'awiya in Syria. I (Fadl) arrived in Syria, and did the needful for her. It was there in Syria that the month of Ramadan commenced. I saw the new moon (of Ramadan) on Friday. I then came back to Medina at the end of the month. Abdullah b. 'Abbas (Allah be pleased with him) asked me (about the new moon of Ramadan) and said: When did you see it? I said.: We saw it on the night of Friday. He said: (Did) you see it yourself? I said: Yes, and the people also saw it and they observed fast and Mu'awiya also observed fast, whereupon he said: But we saw it on Saturday night. So we would continue to observe fast till we complete thirty (lasts) or we see it (the new moon of Shawwal). I said: Is the sighting of the moon by Mu'awiya not valid for you? He said: No;
this is how the Messenger of Allah ﷺ has commanded us. Yahya b. Yahya was in doubt (whether the word used in the narration by Kuraib) was Naktafi or Taktafi.

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، وَيَحْيَى بْنُ أَيُّوبَ، وَقُتَيْبَةُ، وَابْنُ، حُجْرٍ قَالَ يَحْيَى بْنُ يَحْيَى أَخْبَرَنَا وَقَالَ الآخَرُونَ، حَدَّثَنَا إِسْمَاعِيلُ، - وَهُوَ ابْنُ جَعْفَرٍ - عَنْ مُحَمَّدٍ، - وَهُوَ ابْنُ أَبِي حَرْمَلَةَ - عَنْ كُرَيْبٍ، أَنَّ أُمَّ الْفَضْلِ بِنْتَ الْحَارِثِ، بَعَثَتْهُ إِلَى مُعَاوِيَةَ بِالشَّامِ قَالَ فَقَدِمْتُ الشَّامَ فَقَضَيْتُ حَاجَتَهَا وَاسْتُهِلَّ عَلَىَّ رَمَضَانُ وَأَنَا بِالشَّامِ فَرَأَيْتُ الْهِلاَلَ لَيْلَةَ الْجُمُعَةِ ثُمَّ قَدِمْتُ الْمَدِينَةَ فِي آخِرِ الشَّهْرِ فَسَأَلَنِي عَبْدُ اللَّهِ بْنُ عَبَّاسٍ - رضى الله عنهما - ثُمَّ ذَكَرَ الْهِلاَلَ فَقَالَ مَتَى رَأَيْتُمُ الْهِلاَلَ فَقُلْتُ رَأَيْنَاهُ لَيْلَةَ الْجُمُعَةِ . فَقَالَ أَنْتَ رَأَيْتَهُ فَقُلْتُ نَعَمْ وَرَآهُ النَّاسُ وَصَامُوا وَصَامَ مُعَاوِيَةُ . فَقَالَ لَكِنَّا رَأَيْنَاهُ لَيْلَةَ السَّبْتِ فَلاَ نَزَالُ نَصُومُ حَتَّى نُكْمِلَ ثَلاَثِينَ أَوْ نَرَاهُ . فَقُلْتُ أَوَلاَ تَكْتَفِي بِرُؤْيَةِ مُعَاوِيَةَ وَصِيَامِهِ فَقَالَ لاَ هَكَذَا أَمَرَنَا رَسُولُ اللَّهِ ﷺ . وَشَكَّ يَحْيَى بْنُ يَحْيَى فِي نَكْتَفِي أَوْ تَكْتَفِي .