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 Messenger of Allah ﷺ on a journey. He said to a man, "Get down and mix Sawiq (powdered barley) with water for me." The man said, "The sun (has not set yet), O Messenger of Allah ﷺ." The Prophet ﷺ again said to him, "Get down and mix Sawiq with water for me." The man again said, "O Messenger of Allah ﷺ! The sun!" The Prophet ﷺ said to him (for the third time) "Get down and mix Sawiq with water for me." The man dismounted and mixed Sawiq with water for him. The Prophet ﷺ drank it and then beckoned with his hand (towards the East) and said, "When you see the night falling from this side, then a fasting person should break his fast."  

كُنَّا مَعَ رَسُولِ اللَّهِ ﷺ فِي سَفَرٍ فَقَالَ لِرَجُلٍ انْزِلْ فَاجْدَحْ لِي قَالَ يَا رَسُولَ اللَّهِ الشَّمْسُ قَالَ انْزِلْ فَاجْدَحْ لِي قَالَ يَا رَسُولَ اللَّهِ الشَّمْسُ قَالَ انْزِلْ فَاجْدَحْ لِي فَنَزَلَ فَجَدَحَ لَهُ فَشَرِبَ ثُمَّ رَمَى بِيَدِهِ هَا هُنَا ثُمَّ قَالَ إِذَا رَأَيْتُمُ اللَّيْلَ أَقْبَلَ مِنْ هَا هُنَا فَقَدْ أَفْطَرَ الصَّائِمُ