Sahih Muslim Book 008, Hadith Number 3237.

Chapter: One who can support a wife should marry.

Sa'd b. Abi Waqqas (Allah be pleased with him) reported: The Messenger of Allah (May peace be upon him) rejected (the idea) of Uthman b. Muz' un-living in celibacy (saying): And if he (the Holy Prophet) had given me permission We would have got ourselves castrated.