Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) started the race of the conditioned horses from al-Hafya', while the race of the unconditioned horses ran from al-Thaniyyah to the mosque of Banu Zurayq.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السبق والرمي / 19766