Chapter 1: Purification (Kitab Al-Taharah)
Narrated Ibn 'Umar: A man passed by the Prophet (sal Allaahu alayhi wa sallam ) while he was urinating, and saluted him. The Prophet (sal Allaahu alayhi wa sallam) did not return the salutation to him. Abu Dawud said: It is narrated on the authority of Ibn 'Umar that the Prophet (sal Allaahu alayhi wa sallam) performed tayammum, then he returned the salutation to the man.
Sunan Abi Dawud 16