Musnad Abu Ya'la al-Mawsili 6:114

Zuhayr narrated to us, Abd al-Rahman ibn Mahdi narrated to us, Ibrahim ibn Sa'd narrated to us, from al-Zuhri, from Sa'id ibn al-Musayyib, from Sa'd, who said: Uthman sought permission from the Messenger of Allah (may Allah bless him and grant him peace) for celibacy. And had he permitted him, we would have castrated ourselves.