Musnad Abu Ya'la al-Mawsili 1:44

Musa ibn Hayyan narrated to us, Ubayd Allah ibn Abd al-Majid al-Hanafi narrated to us, Musa ibn Mutayr narrated to us, my father narrated to me, from Aisha, she said: Abu Bakr narrated to me, he said: A man from the polytheists came until he faced the Messenger of Allah, may Allah bless him and grant him peace, with his private parts, urinating. I said: "O Messenger of Allah, does the man not see us?" He said: "If he saw us, he would not have faced us with his private parts." Meaning while they were in the cave.