Maliks Muwatta Book 21, Hadith Number 45.

Section : Horses and Racing Them and Expending in Military Expeditions.

Yahya related to me from Malik from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, held a race between horses which had been made lean by training, from al-Hafya to Thaniyatul-Wada. He held a race between horses which had not been made lean from the Thaniya (a mountain pass near Madina) to the mosque of the Banu Zurayq. Abdullah ibn Umar was among those who raced them.