Musnad Ahmad 28:136

Hassan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn Abdullah narrated to me, from Abu Abd al-Rahman al-Hubuli, from Abdullah ibn Amr, who said: A man came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "O Messenger of Allah, permit me to castrate myself." The Messenger of Allah, may Allah bless him and grant him peace, said: "The castration of my nation is fasting and standing in prayer."