Musnad Abu Ya'la al-Mawsili 42:163

Abu Musa Ishaq ibn Musa al-Ansari narrated to us, Ibn Abi Fudayk narrated to us, from al-Dahhak ibn Uthman, Zayd ibn Aslam narrated to me, from Abd al-Rahman ibn Abi Sa’id al-Khudri, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) 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. And a man shall not lie with a man under a single garment, and a woman shall not lie with a woman under a single garment."