He greeted the Prophet ﷺ with the salam while he was urinating, but the Prophet did not return the greeting until he had performed ablution. Once he had performed ablution, he returned the greeting.
أخبرنا محمد بن بشار قال حدثنا معاذ بن معاذ قال أنبأنا سعيد عن قتادة عن الحسن عن حضين أبي ساسان
أنه سلم على النبي ﷺ وهو يبول فلم يرد عليه حتى توضأ فلما توضأ رد عليه
Its isnad is daif. It was also related by Abu Dawud, Book of Purification, chapter, "Should a man return a greeting while he is urinating?", hadith (17), and Ibn Majah, Book of Purification, chapter, "A man is greeted while he is urinating", hadith (350), from the hadith of Saeed bin Abu Aruba with the same chain. Ibn Khuzayma graded it sahih (1/103), as did Ibn Hibban in Mawarid (189, 190), and Hakim, saying it meets the standard of the two Shaykhs (1/167), and Dhahabi agreed with him. It also appears in Sunan al-Kubra (37). Hasan used "an" without stating the direct hearing (36), and the hadith has corroborating narrations.