Sahih Muslim 1440
صحيح مسلم#1440SahihSalama 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)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 5:273