Musannaf Ibn Abi Shayba 28:649

'Abd al-Rahim ibn Sulayman narrated to us, from Juwaybir, who said: 'Umar ibn 'Abd al-'Aziz wrote to us while we were in Khurasan: "Reliable information has reached us from the Messenger of Allah (may Allah bless him and grant him peace) that he allotted three shares for the horseman: two shares for his horse and one share for him, and he allotted one share for the foot soldier." And he said regarding horses: "The Arabian, the crossbred, and the pack horses are all the same."