Musnad Ahmad 33:988

Rawh narrated to us, Husayn al-Mu‘allim narrated to us, from Yahya ibn Abi Kathir, from a man, from Jabir ibn ‘Abd Allah, that a young man came to the Prophet, may Allah bless him and grant him peace, asking his permission for castration. He said: "Fast, and ask Allah for His bounty."