The Prophet of Allah ﷺ used to train horses by reducing their feed and then race them.
أن نبي الله ﷺ كان يضمر الخيل يسابق بها.
Abu Dawud (2576) narrated it from Musaddad, who said, Mutamir related to us, from Ubaydullah, from Nafi, from Ibn Umar, and he mentioned it. This is a sahih chain.Conditioning horses means reducing their feed for a period, putting them in a covered shelter, and covering them with blankets so they sweat, then their sweat dries and their flesh becomes lean, so they become fit for running. Nawawi said this in his commentary on Muslim.And "kinn" refers to structures and dwellings that shield from heat and cold.