Musannaf Ibn Abi Shayba 13:743

Abu Bakr narrated to us, he said: Waki' narrated to us, from Sufyan, from Khalid ibn Salamah, he said: A man came to Umar while he was at Arafat and said: "I have emancipated one-third of my slave." Umar said: "He is entirely free; he has no partner."