Sahih Muslim | Book 8 | Hadith Number 3239

Narrated by Sa'id b. al Musayyib
Sa'id b. al Musayyib heard Sa'd b. Abi Waqqas (Allah be pleased with him) saying that Uthman b. Maz'un decided to live in celibacy, but Allah's Messenger (may peace be upon him) forbade him to do so, and if he had permitted him, we would have got ourselves castrated.