Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that he was riding a horse of his on the day the Muslims met (the tribes of) Tayyi' and Asad, and the commander of the Muslims was Sayyiduna Khalid bin al-Walid (may Allah be pleased with him), whom Sayyiduna Abu Bakr (may Allah be pleased with him) had appointed. The horse plunged with Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) into a ditch and threw him; the horse ran off and the enemy seized it. When Allah Almighty defeated the enemy, Sayyiduna Khalid bin al-Walid (may Allah be pleased with him) returned the horse to Sayyiduna Abdullah bin Umar (may Allah be pleased with them both).
Note: It is possible that the slave was the one returned to him in the time of the Prophet (peace and blessings be upon him) and the horse after him, so that it agrees with the narration of Yahya bin Zakariyya bin Abi Za'idah and then with this narration of Musa bin Uqbah, and Allah knows best. None of these narrations mentions the matter of division; perhaps in Yahya's narration it is the words of one of the narrators rather than of Ibn Umar.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18249