Ibn 'Abbaas Radiyallahu 'Anhu relates:
"Rasulullah Sallallahu 'Alayhi Wasallam once, after relieving himself in the toilet, came out, and was served food. The Sahaabah inquired: 'Will you not perform wudu?'. Rasulullah Saliallahu 'Alayhi Wasallam replied: 'Do I have to perform salaah, that wudu must be performed?'"
English reference: Book 26, Hadith 177
Arabic reference: Book 27, Hadith 186