muhaajir b. qunfudth, with whom allah is pleased, reported that he passed by the prophet , may allah exalt his mention, while he was urinating; he greeted him with salaam, but the prophet, may allah exalt his mention, did not return the greeting until he made wudhu and apologized saying:

'i disliked that i should mention allah's name while i am not in a state of purity.'  (ibn khuzaimah #206)