Musannaf Ibn Abi Shayba 1:1131

'Abdah ibn Sulayman narrated to us, from Yahya ibn Sa'id, from 'Abdullah ibn 'Amir, who said: My father saw me and a man bathing; I was pouring [water] upon him and he was pouring [water] upon me. He shouted at us and said: "Does a man look at the 'awrah (private parts) of a man? By Allah, I see you both [as if you are] the successors."