Sahih Muslim Book 008, Hadith Number 3238.

Chapter: One who can support a wife should marry.

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.