Musnad Ahmad 8:76

Hajjaj narrated to us, Layth informed us, Uqayl narrated to me, from Ibn Shihab, Sa'id ibn al-Musayyib informed me that he heard Sa'd ibn Abi Waqqas say: "Uthman ibn Maz'un wanted to practice celibacy, but the Messenger of Allah (peace and blessings of Allah be upon him) forbade him from doing so." And had he permitted that for him, we would have castrated ourselves.