Musnad Abu Ya'la al-Mawsili 110:171

Abu Bakr narrated to us, Ubaydullah ibn Musa narrated to us, Ibn Abi Dhi'b narrated to us, from al-Zuhri, from Ubaydullah ibn Abdullah ibn Utbah, from Ibn Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) forbade the *sabr* (imprisonment/restraint) of a living soul. Al-Zuhri said: Castration is a severe *sabr*.