Musannaf Ibn Abi Shayba 1:1136

Zayd ibn al-Hubab narrated to us, from al-Dahhak ibn Uthman, who said: Zayd ibn Aslam informed me, from Abd al-Rahman ibn Abi Sa'id al-Khudri, from his father, that the Messenger of Allah (may Allah bless him and grant him peace) said: "A man shall not look at the private parts of a man, and a woman shall not look at the private parts of a woman."