كتاب الإمارة
33  
The Book on Government
(26)
Chapter: The virtue of horses; "Goodness is tied to their Forelocks."
(26)
باب الْخَيْلُ فِي نَوَاصِيهَا الْخَيْرُ إِلَى يَوْمِ الْقِيَامَةِ ‏‏
Sahih Muslim 1873 a

The same tradition has been narrated on the authority of Urwat al-Bariqi who said that the Prophet (ﷺ) said:

Great good is attached to the forelock of the horses until the Day of Judgment.
وَحَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا زَكَرِيَّاءُ، عَنْ عَامِرٍ، عَنْ عُرْوَةَ الْبَارِقِيِّ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ الْخَيْلُ مَعْقُودٌ فِي نَوَاصِيهَا الْخَيْرُ إِلَى يَوْمِ الْقِيَامَةِ الأَجْرُ وَالْمَغْنَمُ ‏"‏ ‏.‏
Reference : Sahih Muslim 1873a
In-book reference : Book 33, Hadith 145
USC-MSA web (English) reference : Book 20, Hadith 4616
  (deprecated numbering scheme)