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. (Using translation from Abū Dāʾūd 4107)
كَانَ رَجُلٌ يَدْخُلُ عَلَى أَزْوَاجِ النَّبِيِّ ﷺ مُخَنَّثٌ وَكَانُوا يَعُدُّونَهُ مِنْ غَيْرِ أُولِيَ الْإِرْبَةِ فَدَخَلَ النَّبِيُّ ﷺ يَوْمًا وَهُوَ عِنْدَ بَعْضِ نِسَائِهِ وَهُوَ يَنْعَتُ امْرَأَةً فَقَالَ إِنَّهَا إِذَا أَقْبَلَتْ أَقْبَلَتْ بِأَرْبَعٍ وَإِذَا أَدْبَرَتْ أَدْبَرَتْ بِثَمَانٍ فَقَالَ النَّبِيُّ ﷺ لَا أَرَى هَذَا يَعْلَمُ مَا هَاهُنَا لَا يَدْخُلْ عَلَيْكُنَّ هَذَا فَحَجَبُوهُ