Musnad Ahmad 8:149

Abu Kamil narrated to us, Ibrahim ibn Sa'd narrated to us, Ibn Shihab narrated to us, from Sa'id ibn al-Musayyib, who said: I heard Sa'd ibn Abi Waqqas saying: "The Messenger of Allah, may Allah bless him and grant him peace, indeed rejected celibacy for Uthman ibn Maz'un." And if he had permitted it for him, we would have castrated ourselves.