(38) باب بَيَانِ أَنَّ أَوَّلَ وَقْتِ الْمَغْرِبِ عِنْدَ غُرُوبِ الشَّمْسِ
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).
Reference: Sahih Muslim 636
In-book reference: Book 5, Hadith 276
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1328