Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) narrates that he greeted the Prophet (peace and blessings be upon him) with salam while he was urinating. The Prophet (peace and blessings be upon him) did not return his greeting until he had finished.
Imam al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains a narrator whom I do not know.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1505