We used to offer the Maghrib prayer with the Prophet ﷺ and then shoot arrows, one of us could see the place where arrow would fall.  

كُنَّا نُصَلِّي الْمَغْرِبَ مَعَ النَّبِيِّ ﷺ ثُمَّ نَرْمِي فَيَرَى أَحَدُنَا مَوْضِعَ نَبْلِهِ  

The Prophet ﷺ used to say the Maghrib prayer immediately after the sun had set when its upper side would disappear.  

كَانَ النَّبِيُّ ﷺ يُصَلِّي الْمَغْرِبَ سَاعَةَ تَغْرُبُ الشَّمْسُ إِذَا غَابَ حَاجِبُهَا  

Marthad ibn Abdullah said: When AbuAyyub came upon us to fight the infidels and in those days Uqbah ibn Amir was the Governor of Egypt, he (Uqbah) delayed the sunset prayer. Hence AbuAyyub stood and said: What kind of prayer is this, Uqbah? He said: We were busy. He said: Did you not hear the Messenger of Allah ﷺ say: My community will remain well, or he said: will remain on its natural condition, so long as it would not delay the evening prayer until the stars shine brightly just like a network.  

لَهُ مَا هَذِهِ الصَّلاَةُ يَا عُقْبَةُ فَقَالَ شُغِلْنَا قَالَ أَمَا سَمِعْتَ رَسُولَ اللَّهِ ﷺ يَقُولُ لاَ تَزَالُ أُمَّتِي بِخَيْرٍ أَوْ قَالَ عَلَى الْفِطْرَةِ مَا لَمْ يُؤَخِّرُوا الْمَغْرِبَ إِلَى أَنْ تَشْتَبِكَ النُّجُومُ