Narrated Aishah, Ummul Muminin: 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.
Hadith Referenceسنن ابي داود / كتاب اللباس / 4107
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:صحيح مسلم (2181)
Benefits and Issues: In Surah An-Nur, the verse of hijab (31), among those from whom there is no need to observe hijab, (ghayru uli al-irbah) are also mentioned, meaning such elderly men who no longer have any desire for women. However, despite their age, if it is sensed that intellectually this man is still interested in women, then observing hijab from him is obligatory. It is not difficult to discern a person’s temperament and inclinations from his conversation and manner of sitting and rising, as is mentioned in the following hadith.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 4107