It is narrated from Ubayy bin Ka'b (may Allah be pleased with him): It is part of the trust (amanah) that a woman is the trustee of her private parts. And Imam al-Shafi'i (may Allah have mercy on him) narrated from Sufyan, from Amr bin Dinar, that Ubayd bin Umayr said: The woman has been entrusted with her private parts.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15404