Abu Dawud Book 27, Hadith Number 4095.

Chapter : Meaning of the verse …or male servents free of physical needs.

Narated By ‘Aisha, Ummul Mu’minin : A mukhannath (eunuch) used to enter upon the wives of Prophet (PBUH). They (the people) counted him among those who were free of physical needs. One day the Prophet (PBUH) entered upon us when he was with one of his wives, and was describing the qualities of a woman, saying: When she comes forward, she comes forward with four (folds in her stomach), and when she goes backward, she goes backward with eight (folds in her stomach). The Prophet (PBUH) said: Do I not see that this (man) knows what here lies. Then they (the wives) observed veil from him.