Umar bin Khattab wrote to Abu Ubayda, "To proceed. It has reached me that some Muslim women enter the bathhouses together with women of the polytheists. Forbid this among those under your authority, for it is not lawful for a woman who believes in Allah and the Last Day to allow her nakedness to be seen by anyone outside her own faith."
كتب عمر بن الخطاب إلى أبي عبيدة: أما بعد! فإنه بلغني أن نساء من نساء المسلمين يدخلن الحمامات مع نساء أهل الشرك فإنه من قبلك عن ذلك، فإنه لا يحل لامرأة تؤمن بالله واليوم الآخر أن ينظر إلى عورتها إلا أهل ملتها.
No references are available.
Bayhaqi reported it in al-Kubra (7/95) from Abu Nasr, who said, Abu Mansur informed us, who said, Ahmad (Ibn Najda) narrated to us, who said, Saeed (Ibn Mansur) narrated to us, who said, Ismail bin Ayyash narrated to us, from Hisham bin Ghaz, from Ubada bin Nusai, from his father, from Harith bin Qays, and he related it.This chain contains Nusai, in the diminutive form, al-Kindi al-Shami, from whom no one narrated except his son Ubada. Ibn Hibban mentioned him in "al-Thiqat," but he is majhul, since no one besides Ibn Hibban attested to his reliability. However, this is the tafsir relied upon by the early scholars, that they forbade Muslim women from displaying their adornment to non-Muslim women, whether from the People of the Book or from shirk. Hafiz bin Kathir mentioned it in his tafsir and remained silent about it.His statement, "to her private area," refers to the lesser private area, which is anything other than the front and back private parts, as has been reported from some scholars. As for the greater private area, meaning the front and back private parts, it is not permissible to look at them, for there is a prohibition against a woman looking at another woman's private area, and likewise a man looking at another man's private area.