Sahih Bukhari Volume 1, Book 7, Hadith Number 333

Narated By Abu Juhaim Al-Ansari : The Prophet came from the direction of Bir Jamal. A man met him and greeted him. But he did not return back the greeting till he went to a (mud) wall and smeared his hands and his face with its dust (performed Tayammum) and then returned back the greeting.