Musnad Ahmad 1153:1167

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from 'Urwah ibn al-Zubayr, from 'Aishah, she said: A mukhannath (effeminate man) used to enter upon the wives of the Prophet, may Allah bless him and grant him peace, and they used to count him among those who have no sexual desire. One day, the Prophet, may Allah bless him and grant him peace, entered while he was with some of his wives, and he was describing a woman. He said: "If she approaches, she approaches with four, and if she turns away, she turns away with eight." The Prophet, may Allah bless him and grant him peace, said: "I do not see that this one knows what is here. Let this one not enter upon you." So they veiled him.