The Prophet ﷺ as saying: When the night approaches from this side and the day retreats on that side, and the sun sets - according to the version of Musaddad - he who fasts has reached the time to break it.  

قَالَ النَّبِيُّ ﷺ إِذَا جَاءَ اللَّيْلُ مِنْ هَا هُنَا وَذَهَبَ النَّهَارُ مِنْ هَا هُنَا زَادَ مُسَدَّدٌ وَغَابَتِ الشَّمْسُ فَقَدْ أَفْطَرَ الصَّائِمُ  

We went along with the Messenger of Allah ﷺ while he was fasting. When the sun set, he said to Bilal: Bilal, come down and prepare barley beverage for us. He said: Messenger of Allah, would that you waited for the evening. He said: Come down and prepare barley beverage for us. He said: Messenger of Allah, the say still remains on you (i.e. there remains the brightness of the day). He said: Come down and prepare barley drink for us. So he came down and prepared barley drink. The Messenger of Allah ﷺ drank it and said: When you see that the night approaches from this side, he who fasts has reached the time to break it ; and he pointed to the east with his finger.  

سَمِعْتُ عَبْدَ اللَّهِ بْنَ أَبِي أَوْفَى يَقُولُ سِرْنَا مَعَ رَسُولِ اللَّهِ ﷺ وَهُوَ صَائِمٌ فَلَمَّا غَرَبَتِ الشَّمْسُ قَالَ يَا بِلاَلُ انْزِلْ فَاجْدَحْ لَنَا قَالَ يَا رَسُولَ اللَّهِ لَوْ أَمْسَيْتَ قَالَ انْزِلْ فَاجْدَحْ لَنَا قَالَ يَا رَسُولَ اللَّهِ إِنَّ عَلَيْكَ نَهَارًا قَالَ انْزِلْ فَاجْدَحْ لَنَا فَنَزَلَ فَجَدَحَ فَشَرِبَ رَسُولُ اللَّهِ ﷺ ثُمَّ قَالَ إِذَا رَأَيْتُمُ اللَّيْلَ قَدْ أَقْبَلَ مِنْ هَا هُنَا فَقَدْ أَفْطَرَ الصَّائِمُ وَأَشَارَ بِأُصْبُعِهِ قِبَلَ الْمَشْرِقِ