Sahih Muslim 636
Sahih Muslim · Book 5: The Book of Mosques and Places of Prayer · The beginning of the time for Maghrib when the sun sets
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، حَدَّثَنَا حَاتِمٌ، - وَهُوَ ابْنُ إِسْمَاعِيلَ - عَنْ يَزِيدَ بْنِ أَبِي عُبَيْدٍ ، عَنْ سَلَمَةَ بْنِ الأَكْوَعِ ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يُصَلِّي الْمَغْرِبَ إِذَا غَرَبَتِ الشَّمْسُ وَتَوَارَتْ بِالْحِجَابِ .
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).