Musnad Ahmad 31:610

Muhammad ibn Isma'il ibn Abi Fudayk narrated to us, al-Dahhak—meaning ibn 'Uthman—narrated to us, from Zayd ibn Aslam, from 'Abd al-Rahman ibn Abi Sa'id, from his father, that the Prophet (peace and blessings of Allah be upon him) said: "A man shall not look at the 'awrah (private parts) of a man, and a woman shall not look at the 'awrah 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."