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." (ID)
English reference (ID): Vol. 1, Book 1, Hadith 661 (ID)
Arabic reference (ID): Book 1, Hadith 706 (ID)