Language:
كتاب الجهاد والسير
56  
Fighting for the Cause of Allah (Jihaad)
(57)
Chapter: Idmar (the preparation) of horses for racing
(57)
باب إِضْمَارِ الْخَيْلِ لِلسَّبْقِ
Sahih al-Bukhari 2869

Narrated `Abdullah:

The Prophet (ﷺ) arranged for a horse race of the horses which had not been made lean; the area of the race was from Ath-Thaniya to the mosque of Bani Zuraiq. (The sub-narrator said, "`Abdullah bin `Umar was amongst those who participated in that horse race.").

حَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ، حَدَّثَنَا اللَّيْثُ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه أَنَّ النَّبِيَّ صلى الله عليه وسلم سَابَقَ بَيْنَ الْخَيْلِ الَّتِي لَمْ تُضَمَّرْ، وَكَانَ أَمَدُهَا مِنَ الثَّنِيَّةِ إِلَى مَسْجِدِ بَنِي زُرَيْقٍ‏.‏ وَأَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ كَانَ سَابَقَ بِهَا‏.‏

قَالَ أَبُو عَبْد اللَّهِ أَمَدًا غَايَةً فَطَالَ عَلَيْهِمْ الْأَمَدُ

Reference : Sahih al-Bukhari 2869
In-book reference : Book 56, Hadith 85
USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 121
  (deprecated numbering scheme)