Musannaf Ibn Abi Shayba 13:618

Abu Bakr narrated to us, he said: Ibn Abi Za'idah [and Waki'] narrated to us, from al-Mas'udi, from al-Qasim, from Jabir ibn Samurah, he said: Umar said: "You are entering into mukatabah (contracts of manumission) with your mukatabs (slaves under contract). So, when he has paid half, there is no return for him into slavery."