كتاب الصلاة
2  
The Book of Prayer
Bulugh al-Maram 362
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."
وَلِمُسْلِمٍ عَنْ أَنَسٍ] قَالَ [ : { كُنَّا نُصَلِّي رَكْعَتَيْنِ بَعْدَ غُرُوبِ اَلشَّمْسِ , فَكَانَ ‏- صلى الله عليه وسلم ‏-يَرَانَا , فَلَمْ يَأْمُرْنَا وَلَمْ يَنْهَانَا } 1
Reference : Bulugh al-Maram 362
In-book reference : Book 2, Hadith 362
English translation : Book 2, Hadith 357
Hadith Text
Reference