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

Sahih Muslim · Book 13 · Hadith 1087

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…

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…