Musnad Ahmad 33:919

Ibrahim, meaning Ibn Khalid, narrated to us, Rabah narrated to us, from Ma'mar, from Yahya ibn Abi Kathir, who said: A man narrated to me, from Jabir ibn Abdullah al-Ansari, who said: A young man came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "Will you permit me to be castrated?" He said: "Fast, and ask Allah for His bounty."