Musnad Ahmad 27:2006

'Uqbah Abu Mas'ud al-Mujaddar narrated to us, 'Ubaydullah narrated to us, from Nafi', from Ibn 'Umar: "The Messenger of Allah (ﷺ) held a horse race, and he favored the horses that had reached the age of maturity (al-qarh) in the distance."