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 narrated it in the Book of Menstruation (370) via Sufyan, from Dahhak bin Uthman, from Nafi, from Ibn Umar, and he related it. This is an abridged hadith, and it will come later in the discussion of tayammum that he performed tayammum and returned his greeting.