Musannaf Ibn Abi Shayba 13:2244

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Thawr narrated to us, from Yunus ibn Sayf, from Hizam ibn Hakim, he said: Umar ibn al-Khattab wrote to Umayr ibn Sa’d: "As for what follows: Indeed, let those who are with you from among the Muslims make contracts of manumission (kitabah) with their slaves, so that they may be spared from begging from the people."