Musannaf Ibn Abi Shayba 27:857

Waki’ narrated to us, he said: Abu al-Durays ‘Uqbah ibn ‘Ammar al-‘Absi narrated to us, from Mas’ud ibn Hirash, the brother of Rib’i ibn Hirash, that ‘Umar ibn al-Khattab asked the ‘Absis: "Which of the horses have you found to be the most enduring in your war?" They said: "The dark bay (al-kumayt)."