Musannaf Ibn Abi Shayba 7:94

Muhammad ibn Bakr narrated to us, from Ibn Jurayj, who said: I said to ‘Ata’, "Is it permissible regarding the killing of a soul [to free] a neck of a believer that is not sound, but he benefits from it, such as one who is lame or one who has a paralyzed limb?" He refused [that], and he preferred [that the slave be] sound.