It is narrated from Sayyiduna Sa'id bin al-As (may Allah be pleased with him) that Sayyiduna Uthman bin Maz'un (may Allah be pleased with him) said: "O Messenger of Allah, permit me to castrate myself." He (peace and blessings be upon him) said to him: "O Uthman, Allah has given us, in place of monasticism, the easy and tolerant Hanifiyyah, and saying the takbir upon every height. If you are one of us, then do as we do."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5498