بُلُوغُ الْمَرَامِ
Bulugh al-Maram 362
The Book of Prayer
وَلِمُسْلِمٍ عَنْ أَنَسٍ] قَالَ [ : { كُنَّا نُصَلِّي رَكْعَتَيْنِ بَعْدَ غُرُوبِ اَلشَّمْسِ , فَكَانَ - صلى الله عليه وسلم -يَرَانَا , فَلَمْ يَأْمُرْنَا وَلَمْ يَنْهَانَا } [1]
Muslim has reported from Anas (RA) that he said, "We used to pray two Rak'at after sunset and the Prophet (ﷺ) used to see us, but he neither commanded nor forbade us."
Sunnah.com reference: Book 2, Hadith 266