Musnad Ahmad 32:2045

'Affan narrated to us, Hammad narrated to us, from Thabit, from Anas: "A man was suspected with a woman, so the Prophet, peace and blessings of Allah be upon him, sent Ali to kill him. He found him in a well cooling himself in it. He said to him: 'Give me your hand.' So he gave him his hand, and behold, he was castrated (majub), having no penis. He came to the Messenger of Allah, peace and blessings of Allah be upon him, and informed him. He said: 'By Allah, O Messenger of Allah, he is indeed castrated; he has no penis.'"