Sahih Muslim
The Book of Marriage (Kitab Al-Nikah)
Book Number: 008 / Hadith Number: 3238
Sa'id b. al-Musayyib reported: I heard Sa'd (b. Abi Waqqas) saying that the idea of 'Uthman b. Maz'un for living in celibacy was rejected (by the Holy Prophet), and if he had been given permission they would have got themselves castrated.