Hadithcore
Mishkat al-Masabih, 1236

Abu Huraira said he heard God’s Messenger say, “The most excellent prayer after that which is obligatory is one in the depth of the night.” Ahmad transmitted it.

وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ قَالَ: سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «أَفْضَلُ الصَّلَاةِ بَعْدَ الْمَفْرُوضَةِ صَلَاةٌ فِي جَوف اللَّيْل» . رَوَاهُ أَحْمد

References1 variant
In-Book Reference
Book 4, Hadith 651