The Prophet ﷺ used to say the Maghrib prayer immediately after the sun had set when its upper side would disappear. (Using translation from Abū Dāʾūd 417)  

كَانَ النَّبِيُّ ﷺ «يُصَلِّي الْمَغْرِبَ سَاعَةَ تَغْرُبُ الشَّمْسُ إِذَا غَابَ حَاجِبُهَا»