It was narrated that Ibn 'Umar said:"A man passed by the Prophet while he was urinating and greeted him with the Salam, and he did not return the greeting
Hadith No. 353
Sunan Ibn Majah
Gradings:
Al-Albani:
Hasan Sahih
Muhammad Fouad Abd al-Baqi:
Hasan Sahih
Shuaib Al Arnaut:
Sahih
Zubair Ali Zai:
Sahih Muslim