Musannaf Ibn Abi Shayba 28:652

Yazid ibn Harun narrated to us, he said: Amr ibn Maymun informed us, he said: Umar ibn Abd al-Aziz wrote to the people of al-Jazira: "As to what follows: The shares during the time of the Messenger of Allah (may Allah bless him and grant him peace) were two shares for the horse and one share for the man. I did not think that anyone would intend to diminish a mandatory portion of them until men from those who fight these fortresses did so. So, restore their shares to what they were during the time of the Messenger of Allah (may Allah bless him and grant him peace): two shares for the horse and one share for the man. And how can the shares of the horses be reduced when they are, by the permission of Allah, for their grazing at night, for their outposts by day, and for the pursuit of what they pursue?"