What is the ruling on castrating slaves?

Castration of human beings is forbidden by consensus (ijma'). The evidence for its prohibition is what is established in the Hadith: "Whoever castrates his slave, We will castrate him." Also, because it is an alteration of Allah's creation, which is forbidden. The Prophet, peace and blessings be upon him, rejected the request of those who sought permission for castration and instead guided them to fasting as an alternative.