Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with him) that a man came to the Prophet Muhammad (peace and blessings be upon him) and said: O Messenger of Allah! Will you permit me to be castrated? The Prophet (peace and blessings be upon him) said: "The castration of my Ummah is fasting and standing (in prayer)."