Narrated Aisha, Ummul Mu'minin:

A mukhannath (eunuch) used to enter upon the wives of Prophet (ﷺ). They (the people) counted him among those who were free of physical needs. One day the Prophet (ﷺ) 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 (ﷺ) said: Do I not see that this (man) knows what here lies. Then they (the wives) observed veil from him.

حَدَّثَنَا مُحَمَّدُ بْنُ عُبَيْدٍ، حَدَّثَنَا مُحَمَّدُ بْنُ ثَوْرٍ، عَنْ مَعْمَرٍ، عَنِ الزُّهْرِيِّ، وَهِشَامِ بْنِ عُرْوَةَ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، رضى الله عنها قَالَتْ كَانَ يَدْخُلُ عَلَى أَزْوَاجِ النَّبِيِّ صلى الله عليه وسلم مُخَنَّثٌ فَكَانُوا يَعُدُّونَهُ مِنْ غَيْرِ أُولِي الإِرْبَةِ فَدَخَلَ عَلَيْنَا النَّبِيُّ صلى الله عليه وسلم يَوْمًا وَهُوَ عِنْدَ بَعْضِ نِسَائِهِ وَهُوَ يَنْعَتُ امْرَأَةً فَقَالَ إِنَّهَا إِذَا أَقْبَلَتْ أَقْبَلَتْ بِأَرْبَعٍ وَإِذَا أَدْبَرَتْ أَدْبَرَتْ بِثَمَانٍ ‏.‏ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏

‏ أَلاَ أَرَى هَذَا يَعْلَمُ مَا هَا هُنَا لاَ يَدْخُلَنَّ عَلَيْكُنَّ هَذَا ‏

‏ ‏.‏ فَحَجَبُوهُ ‏.‏