Messenger of Allah ﷺ said, "When night falls from this side and the day vanishes from this side and the sun sets, then the fasting person should break his fast."  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا أَقْبَلَ اللَّيْلُ مِنْ هَا هُنَا وَأَدْبَرَ النَّهَارُ مِنْ هَا هُنَا وَغَرَبَتِ الشَّمْسُ فَقَدْ أَفْطَرَ الصَّائِمُ  

We were in the company of the Prophet ﷺ on a journey and he was fasting, and when the sun set, he addressed somebody, "O so-and-so, get up and mix Sawiq with water for us." He replied, "O Allah's Apostle! (Will you wait) till it is evening?" The Prophet ﷺ said, "Get down and mix Sawiq with water for us." He replied, "O Messenger of Allah ﷺ! (If you wait) till it is evening." The Prophet ﷺ said again, "Get down and mix Sawiq with water for us." He replied, "It is still daytime."(1) The Prophet ﷺ said again, "Get down and mix Sawiq with water for us." He got down and mixed Sawiq for them. The Prophet ﷺ drank it and then said, "When you see night falling from this side, the fasting person should break his fast."  

كُنَّا مَعَ رَسُولِ اللَّهِ ﷺ فِي سَفَرٍ وَهُوَ صَائِمٌ فَلَمَّا غَرَبَتِ الشَّمْسُ قَالَ لِبَعْضِ الْقَوْمِ يَا فُلاَنُ قُمْ فَاجْدَحْ لَنَا فَقَالَ يَا رَسُولَ اللَّهِ لَوْ أَمْسَيْتَ قَالَ انْزِلْ فَاجْدَحْ لَنَا قَالَ يَا رَسُولَ اللَّهِ فَلَوْ أَمْسَيْتَ قَالَ انْزِلْ فَاجْدَحْ لَنَا قَالَ إِنَّ عَلَيْكَ نَهَارًا قَالَ انْزِلْ فَاجْدَحْ لَنَا فَنَزَلَ فَجَدَحَ لَهُمْ فَشَرِبَ النَّبِيُّ ﷺ ثُمَّ قَالَ إِذَا رَأَيْتُمُ اللَّيْلَ قَدْ أَقْبَلَ مِنْ هَا هُنَا فَقَدْ أَفْطَرَ الصَّائِمُ