It was narrated from 'Abdur-Rahman bin Abu Sa'eed Al-Khudri from his father that:
The Messenger of Allah said: "No woman should look at the nakedness of another woman, and no man should look at the nakedness of another man." (ES)
English reference (ES): Vol. 1, Book 1, Hadith 661 (ES)
Arabic reference (ES): Book 1, Hadith 706 (ES)