Musannaf Ibn Abi Shayba 13:2248

Abu Bakr narrated to us, he said: Waki' narrated to us, he said: Sufyan narrated to us, from Jabir, from 'Amir, who said: If he wishes, he may enter into a contract of manumission (kitabah) with his slave, and if he wishes, he may not enter into a contract of manumission with him.