A man passed by while the Messenger of Allah ﷺ was urinating and greeted him with salam, but he did not return the greeting.
أن رجلا مر، ورسول الله ﷺ يبول، فسلم، فلم يرد عليه.
Muslim reported it in the Book of Menstruation (370) from Muhammad bin Abdullah bin Numayr, who said, my father narrated to us, Sufyan narrated to us, from Dahhak bin Uthman, from Nafi, from Ibn Umar, and he mentioned it.