Salama b. al-Akwa' reported that the Messenger of Allah ﷺ used to pray the evening prayer when the sun had set and disappeared (behind the horizon).  

أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ يُصَلِّي الْمَغْرِبَ إِذَا غَرَبَتِ الشَّمْسُ وَتَوَارَتْ بِالْحِجَابِ  

We used to observe the evening prayer with the Messenger of Allah ﷺ and then one of us would go away and he could see the (distant) place where his arrow would fall.  

سَمِعْتُ رَافِعَ بْنَ خَدِيجٍ يَقُولُ كُنَّا نُصَلِّي الْمَغْرِبَ مَعَ رَسُولِ اللَّهِ ﷺ فَيَنْصَرِفُ أَحَدُنَا وَإِنَّهُ لَيُبْصِرُ مَوَاقِعَ نَبْلِهِ  

A hadith like this, i. e." We used to observe evening prayer...." so on and so forth, has been narrated by Rafi' b. Khadij by another chain of transmitters.  

كُنَّا نُصَلِّي الْمَغْرِبَ بِنَحْوِهِ