Sahih Muslim , Book 5, Hadith 276 (In-Book) ⓘ
Salama b. al-Akwa' reported that the Messenger of Allah used to pray the evening prayer when the sun had set and disappeared (behind the horizon).
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 .
also referenced under: Hadith 636 (Reference) & Book 4, Hadith 1328 (USC-MSA)