Sunan an-Nasa'i, Hadith 3572 ⓘ
It was narrated that Jarir said: "I saw the Messenger of Allah twisting the forelock of a horse with his two fingers, and saying: 'Goodness is tied to the forelocks of horses until the Day of Resurrection: Reward and spoils of war.'"
أَخْبَرَنَا عِمْرَانُ بْنُ مُوسَى، قَالَ حَدَّثَنَا عَبْدُ الْوَارِثِ، قَالَ حَدَّثَنَا يُونُسُ، عَنْ عَمْرِو بْنِ سَعِيدٍ، عَنْ أَبِي زُرْعَةَ بْنِ عَمْرِو بْنِ جَرِيرٍ، عَنْ جَرِيرٍ، قَالَ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَفْتِلُ نَاصِيَةَ فَرَسٍ بَيْنَ أُصْبُعَيْهِ وَيَقُولُ " الْخَيْلُ مَعْقُودٌ فِي نَوَاصِيهَا الْخَيْرُ إِلَى يَوْمِ الْقِيَامَةِ الأَجْرُ وَالْغَنِيمَةُ " .
also referenced under: Book 28, Hadith 12 (In-Book) & Volume 4, Book 28, Hadith 3602 (DarusSalam)