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 ‏‏

It has been narrated on the authority of Jarir b. Abdullah who said:

I saw that the Messenger of Allah (ﷺ) was twisting the forelock of a horse with his fingers and he was saying: (A great) benefit. i. e. reward (for rearing them for Jihad) and spoils of war, has been tied to the forelocks of horses until the Day of Judgment.
وَحَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ الْجَهْضَمِيُّ، وَصَالِحُ بْنُ حَاتِمِ بْنِ وَرْدَانَ، جَمِيعًا عَنْ يَزِيدَ، - قَالَ الْجَهْضَمِيُّ حَدَّثَنَا يَزِيدُ بْنُ زُرَيْعٍ، - حَدَّثَنَا يُونُسُ بْنُ عُبَيْدٍ، عَنْ عَمْرِو بْنِ سَعِيدٍ، عَنْ أَبِي زُرْعَةَ بْنِ عَمْرِو بْنِ جَرِيرٍ، عَنْ جَرِيرِ بْنِ عَبْدِ اللَّهِ، قَالَ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَلْوِي نَاصِيَةَ فَرَسٍ بِإِصْبَعِهِ وَهُوَ يَقُولُ ‏ "‏ الْخَيْلُ مَعْقُودٌ بِنَوَاصِيهَا الْخَيْرُ إِلَى يَوْمِ الْقِيَامَةِ الأَجْرُ وَالْغَنِيمَةُ ‏"‏ ‏.‏
Reference : Sahih Muslim 1872 a
In-book reference : Book 33, Hadith 143
USC-MSA web (English) reference : Book 20, Hadith 4614
  (deprecated numbering scheme)