بُلُوغُ الْمَرَامِ
Bulugh al-Maram 369
The Book of Prayer
وَعَنْ أَبِي هُرَيْرَةَ - رضى الله عنه - قَالَ : قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ أَفْضَلُ اَلصَّلَاةِ بَعْدَ اَلْفَرِيضَةِ صَلَاةُ اَللَّيْلِ } أَخْرَجَهُ مُسْلِمٌ . [1]
Narrated Abu Hurairah (RA): Allah's Messenger (ﷺ) said: "The most excellent prayer after that which is obligatory is the (voluntary) late night prayer." [Reported by Muslim].
Sunnah.com reference: Book 2, Hadith 273