Musannaf Ibn Abi Shayba 28:650

'Abd al-Rahim ibn Sulayman narrated to us, from Ash'ath ibn Siwar, from al-Hasan and Ibn Sirin: They both said: When they would go on a military expedition and obtain spoils of war, they would distribute [the shares] for the horseman from the spoils when it was divided: three shares—two shares for his horse and one share for him—and for the foot soldier, one share.