Sunan Abi Dawud, Book 14, Hadith 2570 (USC-MSA) ⓘ
Narrated Abdullah ibn Umar: The Prophet used to make lean by training horses which he employed in the race.
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا مُعْتَمِرٌ، عَنْ عُبَيْدِ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ نَبِيَّ اللَّهِ صلى الله عليه وسلم كَانَ يُضَمِّرُ الْخَيْلَ يُسَابِقُ بِهَا .
also referenced under: Hadith 2576 (DarusSalam) & Hadith 2570 (Hasan)